I try to follow the path of parameters setting from linux user space (arecord/aplay) down to kernel driver. Let's take arecords --period-size as an example.
It all starts in set_params function in aplay.c:
if (period_time > 0)
err = snd_pcm_hw_params_set_period_time_near(handle, params, &period_time, 0);
else
err = snd_pcm_hw_params_set_period_size_near(handle, params, &period_frames, 0);
The function snd_pcm_hw_params_set_period_size_near() is defined in [pcm.c : 5186](alsa-lib https://github.com/alsa-project/alsa-lib/blob/master/src/pcm/pcm.c#L5186), and here my headache starts... This function starts a chain of calls to other functions which doesn't make much sense to me and doesn't seem to be leading to any end call of driver.
There is _end label so I skipped all calls like snd_pcm_hw_param_set_min() or snd_pcm_hw_param_set_max() and went to snd_pcm_hw_param_set_last() hoping for some driver invocation like:
drv->hw_params_set(...);
but instead I found an end call to:
MASK_INLINE unsigned int snd_mask_min(const snd_mask_t *mask)
{
int i;
assert(!snd_mask_empty(mask));
for (i = 0; i < MASK_SIZE; i++) {
if (mask->bits[i])
return ffs(mask->bits[i]) - 1 + (i << 5);
}
return 0;
}
where return values shall be the parameter set.
So to summarize, I found alsa-lib to be very difficult to read and understand. Maybe I am lacking some knowledge though. My question is simple, how is user space parameter passed to kernel driver. Can you provide a software path showing interfaces called?
Thanks.
The hw_params structure contains a configuration space, which is a description of all possible configurations that the device can support. Numeric parameters are described as intervals (i.e., min and max), access and format as bitmasks.
When you change one parameter, the library calls the kernel driver (SNDRV_PCM_IOCTL_HW_REFINE) to adjust all the other parameters in the hw_params structure that depend on the changed parameter.
After you have reduced the configuration space to the configuration you actually want, you call snd_pcm_hw_params() (→ SNDRV_PCM_IOCTL_HW_PARAMS) to actually configure the device for those parameters. (If some parameter has not been reduced to a single value, snd_pcm_hw_params() will choose a random one.)
snd_pcm_hw_params_set_xxx_near() is more complex because there is no SET_NEAR ioctl. This function tries to adjust the interval so that either its maximum or its minimum is the desired value, and then check whether the actual maximum or minimum is nearer.
For example, assume a device that supports period sizes of 1024, 2048, 4096, and 8192 frames. Initially, the interval is described as [1024, 8192]. When you call snd_pcm_hw_params_set_period_size_near(4000), the snd_pcm_hw_param_set_near() helper function calls set_min(4000) and set_max(4000) (on separate copies of the hw_params structure), so the intervals are [1024, 4000] and [4000, 8192]; after refining, the driver returns the intervals [1024, 2048] and [4096, 8192]. snd_pcm_hw_param_set_near() then sees that 4096 is nearest to the desired calue, so it then calls set_first on the second interval, which results in [4096, 4096].
In the code given below, there is a simple Linux kernel module (driver) which calls a function repeatedly 10 times, using add_timer at resolution of 1 jiffy (that is, the timer is scheduled to fire at jiffies + 1). Using the bash script rerun.sh, then I obtain timestamps from printout in syslog, and visualize them using gnuplot.
In most cases, I get a syslog output like this:
[ 7103.055787] Init testjiffy: 0 ; HZ: 250 ; 1/HZ (ms): 4
[ 7103.056044] testjiffy_timer_function: runcount 1
[ 7103.060045] testjiffy_timer_function: runcount 2
[ 7103.064052] testjiffy_timer_function: runcount 3
[ 7103.068050] testjiffy_timer_function: runcount 4
[ 7103.072053] testjiffy_timer_function: runcount 5
[ 7103.076036] testjiffy_timer_function: runcount 6
[ 7103.080044] testjiffy_timer_function: runcount 7
[ 7103.084044] testjiffy_timer_function: runcount 8
[ 7103.088060] testjiffy_timer_function: runcount 9
[ 7103.092059] testjiffy_timer_function: runcount 10
[ 7104.095429] Exit testjiffy
... which results with time series and delta histogram plots like these:
This is, essentially, the quality of timing that I'd expect from the code.
However - every once in a while, I get a capture like:
[ 7121.377507] Init testjiffy: 0 ; HZ: 250 ; 1/HZ (ms): 4
[ 7121.380049] testjiffy_timer_function: runcount 1
[ 7121.384062] testjiffy_timer_function: runcount 2
[ 7121.392053] testjiffy_timer_function: runcount 3
[ 7121.396055] testjiffy_timer_function: runcount 4
[ 7121.400068] testjiffy_timer_function: runcount 5
[ 7121.404085] testjiffy_timer_function: runcount 6
[ 7121.408084] testjiffy_timer_function: runcount 7
[ 7121.412072] testjiffy_timer_function: runcount 8
[ 7121.416083] testjiffy_timer_function: runcount 9
[ 7121.420066] testjiffy_timer_function: runcount 10
[ 7122.417325] Exit testjiffy
... which results with a rendering like:
... and I'm like: "WHOOOOOAAAAAA ... wait a second..." - isn't there a pulse dropped from the sequence? Meaning that add_timer missed a slot, and then fired up the function in the next 4 ms slot?
The interesting thing is, that in running these tests, I have nothing else but a terminal, web browser and a text editor started up - so I cannot really see anything running, that may hog the OS/kernel; and thus, I really cannot see a reason why the kernel would make such a big miss (of an entire jiffy period). When I read about Linux kernel timing, e.g. "The simplest and least accurate of all timers ... is the timer API", I read that "least accurate" as: "don't expect exactly 4 ms period" (as per this example) - and I don't, I'm fine with the variance shown in the (first) histogram; but I don't expect that a whole period will be missed!?
So my question(s) are:
Is this expected behavior from add_timer at this resolution (that a period can occasionally be missed)?
If so, is there a way to "force" add_timer to fire the function at each 4ms slot, as specified by a jiffy on this platform?
Is it possible that I get a "wrong" timestamp - e.g. the timestamp reflecting when the actual "print" to syslog happened, rather than when the function actually fired?
Note that I'm not looking for a period resolution below what corresponds to a jiffy (in this case, 4ms); nor am I looking to decrease the delta variance when the code works properly. So as I see it, I don't have "high resolution timer" demands, nor "hard real-time" demands - I just want add_timer to fire reliably. Would that be possible on this platform, without resorting to special "real-time" configurations of the kernel?
Bonus question: in rerun.sh below, you'll note two sleeps marked with MUSTHAVE; if either of them is left out/commented, OS/kernel freezes, and requires a hard reboot. And I cannot see why - is it really possible that running rmmod after insmod from bash is so fast, that it will conflict with the normal process of module loading/unloading?
Platform info:
$ cat /proc/cpuinfo | grep "processor\|model name\|MHz\|cores"
processor : 0 # (same for 1)
model name : Intel(R) Atom(TM) CPU N450 # 1.66GHz
cpu MHz : 1000.000
cpu cores : 1
$ echo $(cat /etc/issue ; uname -a)
Ubuntu 11.04 \n \l Linux mypc 2.6.38-16-generic #67-Ubuntu SMP Thu Sep 6 18:00:43 UTC 2012 i686 i686 i386 GNU/Linux
$ echo $(lsb_release -a 2>/dev/null | tr '\n' ' ')
Distributor ID: Ubuntu Description: Ubuntu 11.04 Release: 11.04 Codename: natty
Code:
$ cd /tmp/testjiffy
$ ls
Makefile rerun.sh testjiffy.c
Makefile:
obj-m += testjiffy.o
all:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
testjiffy.c:
/*
* [http://www.tldp.org/LDP/lkmpg/2.6/html/lkmpg.html#AEN189 The Linux Kernel Module Programming Guide]
*/
#include <linux/module.h> /* Needed by all modules */
#include <linux/kernel.h> /* Needed for KERN_INFO */
#include <linux/init.h> /* Needed for the macros */
#include <linux/jiffies.h>
#include <linux/time.h>
#define MAXRUNS 10
static volatile int runcount = 0;
static struct timer_list my_timer;
static void testjiffy_timer_function(unsigned long data)
{
int tdelay = 100;
runcount++;
if (runcount == 5) {
while (tdelay > 0) { tdelay--; } // small delay
}
printk(KERN_INFO
" %s: runcount %d \n",
__func__, runcount);
if (runcount < MAXRUNS) {
my_timer.expires = jiffies + 1;
add_timer(&my_timer);
}
}
static int __init testjiffy_init(void)
{
printk(KERN_INFO
"Init testjiffy: %d ; HZ: %d ; 1/HZ (ms): %d\n",
runcount, HZ, 1000/HZ);
init_timer(&my_timer);
my_timer.function = testjiffy_timer_function;
//my_timer.data = (unsigned long) runcount;
my_timer.expires = jiffies + 1;
add_timer(&my_timer);
return 0;
}
static void __exit testjiffy_exit(void)
{
printk(KERN_INFO "Exit testjiffy\n");
}
module_init(testjiffy_init);
module_exit(testjiffy_exit);
MODULE_LICENSE("GPL");
rerun.sh:
#!/usr/bin/env bash
set -x
make clean
make
# blank syslog first
sudo bash -c 'echo "0" > /var/log/syslog'
sleep 1 # MUSTHAVE 01!
# reload kernel module/driver
sudo insmod ./testjiffy.ko
sleep 1 # MUSTHAVE 02!
sudo rmmod testjiffy
set +x
# copy & process syslog
max=0;
for ix in _testjiffy_*.syslog; do
aa=${ix#_testjiffy_};
ab=${aa%.syslog} ;
case $ab in
*[!0-9]*) ab=0;; # reset if non-digit obtained; else
*) ab=$(echo $ab | bc);; # remove leading zeroes (else octal)
esac
if (( $ab > $max )) ; then
max=$((ab));
fi;
done;
newm=$( printf "%05d" $(($max+1)) );
PLPROC='chomp $_;
if (!$p) {$p=0;}; if (!$f) {$f=$_;} else {
$a=$_-$f; $d=$a-$p;
print "$a $d\n" ; $p=$a;
};'
set -x
grep "testjiffy" /var/log/syslog | cut -d' ' -f7- > _testjiffy_${newm}.syslog
grep "testjiffy_timer_function" _testjiffy_${newm}.syslog \
| sed 's/\[\(.*\)\].*/\1/' \
| perl -ne "$PLPROC" \
> _testjiffy_${newm}.dat
set +x
cat > _testjiffy_${newm}.gp <<EOF
set terminal pngcairo font 'Arial,10' size 900,500
set output '_testjiffy_${newm}.png'
set style line 1 linetype 1 linewidth 3 pointtype 3 linecolor rgb "red"
set multiplot layout 1,2 title "_testjiffy_${newm}.syslog"
set xtics rotate by -45
set title "Time positions"
set yrange [0:1.5]
set offsets graph 50e-3, 1e-3, 0, 0
plot '_testjiffy_${newm}.dat' using 1:(1.0):xtic(gprintf("%.3se%S",\$1)) notitle with points ls 1, '_testjiffy_${newm}.dat' using 1:(1.0) with impulses ls 1
binwidth=0.05e-3
set boxwidth binwidth
bin(x,width)=width*floor(x/width) + width/2.0
set title "Delta diff histogram"
set style fill solid 0.5
set autoscale xy
set offsets graph 0.1e-3, 0.1e-3, 0.1, 0.1
plot '_testjiffy_${newm}.dat' using (bin(\$2,binwidth)):(1.0) smooth freq with boxes ls 1
unset multiplot
EOF
set -x; gnuplot _testjiffy_${newm}.gp ; set +x
EDIT: Motivated by this comment by #granquet, I tried to obtain scheduler statistics from /proc/schedstat and /proc/sched_debug, by using dd through call_usermodehelper; note that this most of the time "skips" (that is, a file due to the 7th, or 6th, or Xth run of the function would be missing); but I managed to obtain two complete runs, and posted them in https://gist.github.com/anonymous/5709699 (as I noticed gist may be preferred to pastebin on SO), given the output is kinda massive; the *_11* files log a proper run, the *_17* files log a run with a "drop".
Note I also switched to mod_timer_pinned in the module, and it doesn't help much (the gist logs are obtained with the module using this function). These are the changes in testjiffy.c:
#include <linux/kmod.h> // usermode-helper API
...
char fcmd[] = "of=/tmp/testjiffy_sched00";
char *dd1argv[] = { "/bin/dd", "if=/proc/schedstat", "oflag=append", "conv=notrunc", &fcmd[0], NULL };
char *dd2argv[] = { "/bin/dd", "if=/proc/sched_debug", "oflag=append", "conv=notrunc", &fcmd[0], NULL };
static char *envp[] = {
"HOME=/",
"TERM=linux",
"PATH=/sbin:/bin:/usr/sbin:/usr/bin", NULL };
static void testjiffy_timer_function(unsigned long data)
{
int tdelay = 100;
unsigned long tjnow;
runcount++;
if (runcount == 5) {
while (tdelay > 0) { tdelay--; } // small delay
}
printk(KERN_INFO
" %s: runcount %d \n",
__func__, runcount);
if (runcount < MAXRUNS) {
mod_timer_pinned(&my_timer, jiffies + 1);
tjnow = jiffies;
printk(KERN_INFO
" testjiffy expires: %lu - jiffies %lu => %lu / %lu\n",
my_timer.expires, tjnow, my_timer.expires-tjnow, jiffies);
sprintf(fcmd, "of=/tmp/testjiffy_sched%02d", runcount);
call_usermodehelper( dd1argv[0], dd1argv, envp, UMH_NO_WAIT );
call_usermodehelper( dd2argv[0], dd2argv, envp, UMH_NO_WAIT );
}
}
... and this in rerun.sh:
...
set +x
for ix in /tmp/testjiffy_sched*; do
echo $ix | tee -a _testjiffy_${newm}.sched
cat $ix >> _testjiffy_${newm}.sched
done
set -x ; sudo rm /tmp/testjiffy_sched* ; set +x
cat > _testjiffy_${newm}.gp <<EOF
...
I'll use this post for verbose replying.
#CL.: many thanks for the answer. Good to have it confirmed that it's "possible that your timer function gets called at a later jiffy"; by logging the jiffies, I too realized that the timer function gets called at a later time - and other than that, it doesn't anything "wrong" per se.
Good to know about the timestamps; I wonder if it is possible that: the timer functions hits at the right time, but the kernel preempts the kernel logging service (I believe it's klogd), so I get a delayed timestamp? However, I'm trying to create a "looped" (or rather, periodic) timer function to write to hardware, and I first noted this "drop" by realizing the PC doesn't write data at certain intervals on the USB bus; and given that the timestamps confirm that behavior, it's probably not the problem here (I guess).
I have modified the timer function so it fires relative to the scheduled time of the last timer (my_timer.expires) - again via mod_timer_pinned instead of add_timer:
static void testjiffy_timer_function(unsigned long data)
{
int tdelay = 100;
unsigned long tjlast;
unsigned long tjnow;
runcount++;
if (runcount == 5) {
while (tdelay > 0) { tdelay--; } // small delay
}
printk(KERN_INFO
" %s: runcount %d \n",
__func__, runcount);
if (runcount < MAXRUNS) {
tjlast = my_timer.expires;
mod_timer_pinned(&my_timer, tjlast + 1);
tjnow = jiffies;
printk(KERN_INFO
" testjiffy expires: %lu - jiffies %lu => %lu / %lu last: %lu\n",
my_timer.expires, tjnow, my_timer.expires-tjnow, jiffies, tjlast);
}
}
... and the first few tries, it works impeccably - however, eventually, I get this:
[13389.775508] Init testjiffy: 0 ; HZ: 250 ; 1/HZ (ms): 4
[13389.776051] testjiffy_timer_function: runcount 1
[13389.776063] testjiffy expires: 3272445 - jiffies 3272444 => 1 / 3272444 last: 3272444
[13389.780053] testjiffy_timer_function: runcount 2
[13389.780068] testjiffy expires: 3272446 - jiffies 3272445 => 1 / 3272445 last: 3272445
[13389.788054] testjiffy_timer_function: runcount 3
[13389.788073] testjiffy expires: 3272447 - jiffies 3272447 => 0 / 3272447 last: 3272446
[13389.788090] testjiffy_timer_function: runcount 4
[13389.788096] testjiffy expires: 3272448 - jiffies 3272447 => 1 / 3272447 last: 3272447
[13389.792070] testjiffy_timer_function: runcount 5
[13389.792091] testjiffy expires: 3272449 - jiffies 3272448 => 1 / 3272448 last: 3272448
[13389.796044] testjiffy_timer_function: runcount 6
[13389.796062] testjiffy expires: 3272450 - jiffies 3272449 => 1 / 3272449 last: 3272449
[13389.800053] testjiffy_timer_function: runcount 7
[13389.800063] testjiffy expires: 3272451 - jiffies 3272450 => 1 / 3272450 last: 3272450
[13389.804056] testjiffy_timer_function: runcount 8
[13389.804072] testjiffy expires: 3272452 - jiffies 3272451 => 1 / 3272451 last: 3272451
[13389.808045] testjiffy_timer_function: runcount 9
[13389.808057] testjiffy expires: 3272453 - jiffies 3272452 => 1 / 3272452 last: 3272452
[13389.812054] testjiffy_timer_function: runcount 10
[13390.815415] Exit testjiffy
... which renders like so:
... so, basically I have a delay/"drop" at +8ms slot (which should be #3272446 jiffies), and then have two functions run at the +12ms slot (which would be # 3272447 jiffies); you can even see the label on the plot as "more bolded" because of it. This is better, in the sense of the "drop" sequence now being synchronous to a proper, non-drop sequence (which is as you said: "to avoid that one late timer function shifts all following timer calls") - however, I still miss a beat; and since I have to write bytes to hardware at each beat, so I keep a sustained, constant transfer rate, this unfortunately doesn't help me much.
As for the other suggestion, to "use ten timers" - because of my ultimate goal (write to hardware using a periodic lo-res timer function); I thought at first it doesn't apply - but if nothing else is possible (other than doing some special real-time kernel preparations), then I'll certainly try a scheme where I have 10 (or N) timers (maybe stored in an array) which are fired periodically one after another.
EDIT: just adding leftover relevant comments:
USB transfers are either scheduled in advance (isochronous) or have no timing guarantees (asynchronous). If your device doesn't use isochronous transfers, it's badly misdesigned. – CL. Jun 5 at 10:47
Thanks for the comment, #CL. - "... scheduled in advance (isochronous)..." cleared a confusion I had. I'm (eventually) targeting an FT232, which only has BULK mode - and as long as the bytes per timer hit is low, I can actually "cheat" my way through in "streaming" data with add_timer; however, when I transfer ammount of bytes close to consuming bandwidth, then these "misfires" start getting noticeable as drops. So I was interested in testing the limits of that, for which I need a reliably repetitive "timer" function - is there anything else I could try to have a reliable "timer"? – sdaau Jun 5 at 12:27
#sdaau Bulk transfers are not suitable for streaming. You cannot fix shortcomings in the hardware protocol by using another kind of software timer. – CL. Jun 5 at 13:50
... and as my response to #CL. : I'm aware I wouldn't be able to fix shortcomings; I was more interested in observing these shortcomings - say, if a kernel function makes a periodic USB write, I could observe the signals on a scope/analyzer, and hopefully see in what sense is the bulk mode unsuitable. But first, I'd have to trust that the function can (at least somewhat) reliably repeat at a periodic rate (i.e. "generate" a clock/tick) - and I wasn't aware, until now, that I cannot really trust add_timer at jiffies resolution (as it is capable of relatively easily skipping a whole period). However, it seems that moving to Linux' high-resolution timers (hrtimer) does give me a reliable periodic function in this sense - so I guess that solves my problem (posted in my answer below).
Many thanks for all the comments and answers; they all pointed to things that must be taken into account - but given I'm somewhat of a forever noob, I still needed to do some more reading, before gaining some understanding (I hope a correct one). Also, I couldn't really find anything specific for periodically "ticking" functions - so I'll post a more verbose answer here.
In brief - for a reliable periodic Linux kernel function at a resolution of a jiffy, do not use add_timer (<linux/time.h>), as it may "drop" an entire period; use high-resolution timers (<linux/hrtimer.h>) instead. In more detail:
Is it possible that I get a "wrong" timestamp - ...?
#CL.: The timestamp in the log is the time when that string was printed to the log.
So, maybe it's possible - but it turns out, that's not the problem here:
Is this expected behavior from add_timer at this resolution (that a period can occasionally be missed)?
I guess, it turns out - yes:
If so, is there a way to "force" add_timer to fire the function at each 4ms slot, as specified by a jiffy on this platform?
... and (I guess again), it turns out - no.
Now, the reasons for this are somewhat subtle - and I hope if I didn't get them right, someone will correct me. First of all, the first misconception that I had, was that "a clock is just a clock" (in the sense of: even if it is implemented as computer code) - but that is not quite correct. The kernel basically has to "queue" an "event" somewhere, each time something like add_timer is used; and this request may come from anything really: from any (and all) sort(s) of driver(s), or even possibly userspace.
The problem is that this "queuing" costs - since in addition to the kernel having to handle (the equivalent of) traversing and inserting (and removing) items in an array, it also has to handle timer delays spanning several orders of magnitude (from say milliseconds to maybe 10s of seconds); and the fact that some drivers (like, apparently, those for network protocols) apparently queue a lot of timer events, which are usually cancelled before running - while other types may require a completely different behavior (like in my case - in a periodic function, you expect that most of the time, the event will usually not be cancelled; and you also queue the events one by one). On top of that, the kernel needs to handle this for uniprocessor vs. SMP vs. multiprocessor platforms. Thus, there is a cost-benefit tradeoff involved in implementing timer handling in the kernel.
It turns out, the architecture around jiffies/add_timer is designed to handle the most common devices - and for them, precision at a resolution of a jiffy is not an issue; but this also means that one cannot expect a reliable timer at resolution of a single jiffy with this method. This is also compounded by the fact that the kernel handles these "event queues" by treating them (somewhat) like interrupt service requests (IRQ); and that there are several levels of priority in IRQ handling in the kernel, where higher priority routine can pre-empt a lower priority one (that is: interrupt and suspend a lower priority routine, even if it is being executed at the time - and allow the higher priority routine to go about its business). Or, as previously noted:
#granquet: timers run in soft irq context, which means they have the highest priority and they preempt everything running/runnable on the CPU ... but hardware interrupts which are not disabled when servicing a soft irq. So you might (most probable explanation) get an Hardware interrupt here and there that preempts your timer ... and thus you get an interrupt that is not serviced at the right time.
#CL.: It is indeed possible that your timer function gets called at a later jiffy than what expires what set to. Possible reasons are scheduling delays, other drivers that disable interrupts for too long (graphics and WLAN drivers are usual culprits), or some crappy BIOS executing SMI code.
I now think so, too - I think this could be an illustration of what happens:
jiffies changes to, say, 10000 (== 40000 ms #250 Hz)
Let's say the timer function, (queued by add_timer) is about to start running - but hasn't started running yet
Let's say here, the network card generates (for whatever reason) a hardware interrupt
The hardware interrupt, having a higher priority, triggers the kernel to pre-empt (stop and suspend) the timer function (possibly started by now, and just few instructions in);
That means the kernel now has to reschedule the timer function, to run at a later point - and since one only works with integer operations in the kernel, and time resolution for this kind of event is in jiffies - the best it can do is reschedule it for jiffies+1 (10001 == 40004 ms #250 Hz)
Now the kernel switches the context to the IRQ service routine of the network card driver, and it goes about its business
Let's say the IRQ service routine completes in 200 μs - that means now we're (in "absolute" terms) at 40000.2 ms - however, we are also still at 10000 jiffies
If the kernel now switched the context back to the timer function, it would have completed - without me necessarily noticing the delay;
... however, that will not happen, because the timer function is scheduled for the next jiffy!
So kernel goes about its business (possibly sleeping) for the next approx 3.8 ms
jiffies changes to 10001 (== 40004 ms #250 Hz)
(the previously rescheduled) timer function runs - and this time completes without interruption
I haven't really done a detailed analysis to see if the sequence of events is exactly as described above; but I'm quite persuaded that it is something close - in other words, a resolution problem - especially since the high-resolution timer approach seems to not show this behavior. It would be great indeed, to obtain a scheduler log, and know exactly what happened to cause a pre-empt - but I doubt the roundtrip to userspace, which I attempted in OP edit, in response to #granquet's comment, is the right thing to do.
In any case, going back to this:
Note that I'm not looking for a period resolution below what corresponds to a jiffy (in this case, 4ms); nor am I looking to decrease the delta variance when the code works properly. So as I see it, I don't have "high resolution timer" demands, nor "hard real-time" demands ...
... here was a bad mistake I made - as the analysis above shows, I did have "high resolution" demands! And had I realized that earlier, I may have found relevant reading sooner. Anyways, some relevant docs - even if they don't discuss specifically periodic functions - for me, were:
LDD3: 5.3. Semaphores and Mutexes - (in describing a driver with different demands from here): "no accesses will be made from interrupt handlers or other asynchronous contexts. There are no particular latency (response time) requirements; application programmers understand that I/O requests are not usually satisfied immediately"
Documentation/timers/hrtimers.txt - "The timers.c code is very "tightly coded" around jiffies and 32-bitness assumptions, and has been honed and micro-optimized for a relatively narrow use case (jiffies in a relatively narrow HZ range) for many years - and thus even small extensions to it easily break the wheel concept"
T. Gleixner, D. Niehaus Hrtimers and Beyond: Transforming the Linux Time Subsystems (pdf) - (most detailed, see also diagrams inside) "The Cascading Timer Wheel (CTW), which was implemented in 1997, replaced the original time ordered double linked list to resolve the scalability problem of the linked list's O(N) insertion time... The current approach to timer management in Linux does a good job of satisfying an extremely wide range of requirements, but it cannot provide the quality of service required in some cases precisely because it must satisfy such a wide range of requirements... The timeout related timers are kept in the existing timer wheel and a new subsystem optimized for (high resolution) timer requirements hrtimers was implemented. hrtimers are entirely based on human time (units: nanoseconds)... They are kept in a time sorted, per-CPU list, implemented as a red-black tree."
The high-resolution timer API [LWN.net] - "At its core, the hrtimer mechanism remains the same. Rather than using the "timer wheel" data structure, hrtimers live on a time-sorted linked list, with the next timer to expire being at the head of the list. A separate red/black tree is also used to enable the insertion and removal of timer events without scanning through the list. But while the core remains the same, just about everything else has changed, at least superficially."
Software interrupts and realtime [LWN.net] - "The softirq mechanism is meant to handle processing that is almost — but not quite — as important as the handling of hardware interrupts. Softirqs run at a high priority (though with an interesting exception, described below), but with hardware interrupts enabled. They thus will normally preempt any work except the response to a "real" hardware interrupt... Starting with the 3.0 realtime patch set, though, that capability went away... In response, in 3.6.1-rt1, the handling of softirqs has changed again."
High- (but not too high-) resolution timeouts [LWN.net] - "_poll() and epoll_wait() take an integer number of milliseconds; select() takes a struct timeval with microsecond resolution, and ppoll() and pselect() take a struct timespec with nanosecond resolution. They are all the same, though, in that they convert this timeout value to jiffies, with a maximum resolution between one and ten milliseconds. A programmer might program a pselect() call with a 10 nanosecond timeout, but the call may not return until 10 milliseconds later, even in the absence of contention for the CPU. ... It's a useful feature, but it comes at the cost of some significant API changes._"
One thing clear from the quotes, is that high-resolution timing facilities are still under active development (with API changes) in the kernel - and I was afraid, that maybe I'd have to install a special "real-time patch" kernel. Thankfully, high-resolution timers are seemingly available (and working) in my 2.6.38-16 SMP kernel without any special changes. Below is the listing of the modified testjiffies.c kernel module, which now uses high-resolution timers, but otherwise keeps the same period as determined by jiffies. For testing, I made it loop for 200 times (instead of 10 in the OP); and running the rerun.sh script for some 20-30 times, this is the worst result I got:
The time sequence is now obviously unreadable, but the histogram can still tell us this: taking 0.00435-0.004 (= 0.004-0.00365) = 350 μs for the max deviation, it represents only 100*(350/4000) = 8.75% of the expected period; which I certainly don't have a problem with. Additionally, I never got a drop (or correspondingly, an entire 2*period = 8 ms delay), or a 0 ms delay - the captures I got, are otherwise of the quality shown on the first image in OP. Now, of course I could run a longer test and see more precisely how reliable it is - but this is all the reliability I'd expect/need to see for this simple case; contrast that to the OP, where I'd get a drop in just 10 loops, with the probability of tossing a coin - every second or third run of the rerun.sh script, I'd get a drop - even in context of low OS resource usage!
Finally, note that the source below should have the problem, spotted by #CL.: "Your module is buggy: you must ensure that the timer is not pending before the module is unloaded", fixed (in the context of hrtimer). This seemingly answers my bonus question, as it obviates the need for either of the "MUSTHAVE" sleeps in the rerun.sh script. However, note that as 200 loops # 4 ms take 0.8 s - the sleep between insmod and rmmod is needed if we want a full 200 tick capture (otherwise, on my machine, I get only some 7 ticks captured).
Well, hope I got this right now (at least most if it) - if not, corrections are welcome :)
testjiffy(-hr).c
#include <linux/module.h> /* Needed by all modules */
#include <linux/kernel.h> /* Needed for KERN_INFO */
#include <linux/init.h> /* Needed for the macros */
#include <linux/jiffies.h>
#include <linux/time.h>
#define MAXRUNS 200
#include <linux/hrtimer.h>
static volatile int runcount = 0;
//~ static struct timer_list my_timer;
static unsigned long period_ms;
static unsigned long period_ns;
static ktime_t ktime_period_ns;
static struct hrtimer my_hrtimer;
//~ static void testjiffy_timer_function(unsigned long data)
static enum hrtimer_restart testjiffy_timer_function(struct hrtimer *timer)
{
int tdelay = 100;
unsigned long tjnow;
ktime_t kt_now;
int ret_overrun;
runcount++;
if (runcount == 5) {
while (tdelay > 0) { tdelay--; } // small delay
}
printk(KERN_INFO
" %s: runcount %d \n",
__func__, runcount);
if (runcount < MAXRUNS) {
tjnow = jiffies;
kt_now = hrtimer_cb_get_time(&my_hrtimer);
ret_overrun = hrtimer_forward(&my_hrtimer, kt_now, ktime_period_ns);
printk(KERN_INFO
" testjiffy jiffies %lu ; ret: %d ; ktnsec: %lld \n",
tjnow, ret_overrun, ktime_to_ns(kt_now));
return HRTIMER_RESTART;
}
else return HRTIMER_NORESTART;
}
static int __init testjiffy_init(void)
{
struct timespec tp_hr_res;
period_ms = 1000/HZ;
hrtimer_get_res(CLOCK_MONOTONIC, &tp_hr_res);
printk(KERN_INFO
"Init testjiffy: %d ; HZ: %d ; 1/HZ (ms): %ld ; hrres: %lld.%.9ld\n",
runcount, HZ, period_ms, (long long)tp_hr_res.tv_sec, tp_hr_res.tv_nsec );
hrtimer_init(&my_hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
my_hrtimer.function = &testjiffy_timer_function;
period_ns = period_ms*( (unsigned long)1E6L );
ktime_period_ns = ktime_set(0,period_ns);
hrtimer_start(&my_hrtimer, ktime_period_ns, HRTIMER_MODE_REL);
return 0;
}
static void __exit testjiffy_exit(void)
{
int ret_cancel = 0;
while( hrtimer_callback_running(&my_hrtimer) ) {
ret_cancel++;
}
if (ret_cancel != 0) {
printk(KERN_INFO " testjiffy Waited for hrtimer callback to finish (%d)\n", ret_cancel);
}
if (hrtimer_active(&my_hrtimer) != 0) {
ret_cancel = hrtimer_cancel(&my_hrtimer);
printk(KERN_INFO " testjiffy active hrtimer cancelled: %d (%d)\n", ret_cancel, runcount);
}
if (hrtimer_is_queued(&my_hrtimer) != 0) {
ret_cancel = hrtimer_cancel(&my_hrtimer);
printk(KERN_INFO " testjiffy queued hrtimer cancelled: %d (%d)\n", ret_cancel, runcount);
}
printk(KERN_INFO "Exit testjiffy\n");
}
module_init(testjiffy_init);
module_exit(testjiffy_exit);
MODULE_LICENSE("GPL");
It is indeed possible that your timer function gets called at a later jiffy than what expires what set to.
Possible reasons are scheduling delays, other drivers that disable interrupts for too long (graphics and WLAN drivers are usual culprits), or some crappy BIOS executing SMI code.
If you want to avoid that one late timer function shifts all following timer calls, you have to schedule the respective next timer not relative to the current time (jiffies), but relative to the scheduled time of the last timer (my_timer.expires).
Alternatively, use ten timers that you all schedule at the beginning at jiffies + 1, 2, 3, …
The timestamp in the log is the time when that string was printed to the log.
Your module is buggy: you must ensure that the timer is not pending before the module is unloaded.
My PC has an AMD processor with an ATI 3200 GPU which doesn't support OpenCL. The rest of the codes all running by "Falling back to CPU itself".
I am converting one of the code from CUDA to OpenCL but stuck in some particular part for which there is no exact conversion code in OpenCL. since i have less experience in OpenCL I can't make out this, please suggest me some solution if any of you think will work,
The CUDA code is,
size_t pitch = 0;
cudaError error = cudaMallocPitch((void**)&gpu_data, (size_t*)&pitch,
instances->cols * sizeof(float), instances->rows);
for( int i = 0; i < instances->rows; i++ ){
error = cudaMemcpy((void*)(gpu_data + (pitch/sizeof(float))*i),
(void*)(instances->data + (instances->cols*i)),
instances->cols * sizeof(float) ,cudaMemcpyHostToDevice);
If I remove the pitch value from the above I end up with an problem which doesn't write to the device memory "gpu_data".
Somebody please convert this code to OpenCL and reply. I have converted it to OpenCL, but its not working and the data is not written to "gpu_data". My converted OpenCL code is
gpu_data = clCreateBuffer(context, CL_MEM_READ_WRITE, ((instances->cols)*(instances->rows))*sizeof(float), NULL, &ret);
for( int i = 0; i < instances->rows; i++ ){
ret = clEnqueueWriteBuffer(command_queue, gpu_data, CL_TRUE, 0, ((instances->cols)*(instances->rows))*sizeof(float),(void*)(instances->data + (instances->cols*i)) , 0, NULL, NULL);
Sometimes it runs well for this code and gets stuck in the reading part i.e.
ret = clEnqueueReadBuffer(command_queue, gpu_data, CL_TRUE, 0,sizeof( float ) * instances->cols* 1 , instances->data, 0, NULL, NULL);
overhere. And it gives error like
Unhandled exception at 0x10001098 in CL_kmeans.exe: 0xC000001D: Illegal Instruction.
when break is pressed , it gives:
No symbols are loaded for any call stack frame. The source code cannot be displayed.
while debugging. In the call stack it is displaying:
OCL8CA9.tmp.dll!10001098()
[Frames below may be incorrect and/or missing, no symbols loaded for OCL8CA9.tmp.dll]
amdocl.dll!5c39de16()
I really dont know what it means. someone please help me to rid of this problem.
First of all, in the CUDA code you're doing a horribly inefficient thing to copy the data. The CUDA runtime has the function cudaMemcpy2D that does exactly what you are trying to do by looping over different rows.
What cudaMallocPitch does is to compute an optimal pitch (= distance in byte between rows in a 2D array) such that each new row begins at an address that is optimal for coalescing, and then allocates a memory area as large as pitch times the number of rows you specify. You can emulate the same thing in OpenCL by first computing the optimal pitch and then doing the allocation of the correct size.
The optimal pitch is computed by (1) getting the base address alignment preference for your card (CL_DEVICE_MEM_BASE_ADDR_ALIGN property with clGetDeviceInfo: note that the returned value is in bits, so you have to divide by 8 to get it in bytes); let's call this base (2) find the largest multiple of base that is no less than your natural data pitch (sizeof(type) times number of columns); this will be your pitch.
You then allocate pitch times number of rows bytes, and pass the pitch information to kernels.
Also, when copying data from the host to the device and converesely, you want to use clEnqueue{Read,Write}BufferRect, that are specifically designed to copy 2D data (they are the counterparts to cudaMemcpy2D).
We've been bashing our heads off of this one all morning. We've got some serial lines setup between an embedded linux device and an Ubuntu box. Our reads are getting screwed up because our code usually returns two (sometimes more, sometimes exactly one) message reads instead of one message read per actual message sent.
Here is the code that opens the serial port. InterCharTime is set to 4.
void COMClass::openPort()
{
struct termios tio;
this->fd = -1;
int tmpFD;
tempFD = open( port, O_RDWR | O_NOCTTY);
if (tempFD < 0)
{
cerr<< "the port is not opened"<< port <<"\n";
portOpen = 0;
return;
}
tio.c_cflag = BaudRate | CS8 | CLOCAL | CREAD ;
tio.c_oflag = 0;
tio.c_iflag = IGNPAR;
newtio.c_cc[VTIME] = InterCharTime;
newtio.c_cc[VMIN] = readBufferSize;
newtio.c_lflag = 0;
tcflush(tempFD, TCIFLUSH);
tcsetattr(tempFD,TCSANOW,&tio);
this->fd = tempFD;
portOpen = true;
}
The other end is configured similarly for communication, and has one small section of particular iterest:
while (1)
{
sprintf(out, "\r\nHello world %lu", ++ulCount);
puts(out);
WritePort((BYTE *)out, strlen(out)+1);
sleep(2);
} //while
Now, when I run a read thread on the receiving machine, "hello world" is usually broken up over a couple messages. Here is some sample output:
1: Hello
2: world 1
3: Hello
4: world 2
5: Hello
6: world 3
where number followed by a colon is one message recieved. Can you see any error we are making?
Thank you.
Edit:
For clarity, please view section 3.2 of the Linux Serial Programming HOWTO. To my understanding, with a VTIME of a couple seconds (meaning vtime is set anywhere between 10 and 50, trial-and-error), and a VMIN of 1, there should be no reason that the message is broken up over two separate messages.
I don't see why you are surprised.
You are asking for at least one byte. If your read() is asking for more, which seems probable since you are surprised you aren't getting the whole string in a single read, it can get whatever data is available up to the read() size. But all the data isn't available in a single read so your string is chopped up between reads.
In this scenario the timer doesn't really matter. The timer won't be set until at least one byte is available. But you have set the minimum at 1. So it just returns whatever number of bytes ( >= 1) are available up to read() size bytes.
If you are still experiencing this problem (realizing the question is old), and your code is accurate, you are setting your VTIME and VMIN in the newtio struct, and the rest of the other parameters in the tio struct.