Writing a syscall to count context switches of a process - linux

Question

I have to do a system call to count the voluntary & involuntary context switches of a process. I already know the steps to add a new system call to a linux kernel but i have no clue of where i should start for the context-switch function. Any idea?

Answer 1

If your syscall should only report statistics, you can use context switch counting code that is already in the kernel.
wait3 syscall or getrusage syscall already reports context switch count in struct rusage fields:
struct rusage {
...
long ru_nvcsw; /* voluntary context switches */
long ru_nivcsw; /* involuntary context switches */
};
You can try it by running:
$ /usr/bin/time -v /bin/ls -R
....
Voluntary context switches: 1669
Involuntary context switches: 207
where "/bin/ls -R" is any program.
By searching an "struct rusage" in kernel sources, you can find this accumulate_thread_rusage in kernel/sys.c, which updates rusage struct. It reads from struct task_struct *t; the fields t->nvcsw; and t->nivcsw;:
1477 static void accumulate_thread_rusage(struct task_struct *t, struct rusage *r)
1478 {
1479 r->ru_nvcsw += t->nvcsw; // <<=== here
1480 r->ru_nivcsw += t->nivcsw;
1481 r->ru_minflt += t->min_flt;
1482 r->ru_majflt += t->maj_flt;
Then you should search nvcsw and nivcsw in kernel folder to find how they are updated by kernel.
asmlinkage void __sched schedule(void):
4124 if (likely(prev != next)) { // <= if we are switching between different tasks
4125 sched_info_switch(prev, next);
4126 perf_event_task_sched_out(prev, next);
4127
4128 rq->nr_switches++;
4129 rq->curr = next;
4130 ++*switch_count; // <= increment nvcsw or nivcsw via pointer
4131
4132 context_switch(rq, prev, next); /* unlocks the rq */
Pointer switch_count is from line 4091 or line 4111 of the same file.
PS: Link from perreal is great: http://oreilly.com/catalog/linuxkernel/chapter/ch10.html (search context_swtch)

Answer 2

This already exists: the virtual file /proc/NNNN/status (where NNNN is the decimal process ID of the process you want to know about) contains, among other things, counts of both voluntary and involuntary context switches. Unlike getrusage this allows you to learn the context switch counts for any process, not just children. See the proc(5) manpage for more details.

Answer 3

A process will make a context switch in case of blocking, time quantum expiring or for interrupts etc. Eventually schedule() function is called. Since you want to count it for each process separately you have to keep a new variable for each process for counting the no of context switches. And you can update this variable each time in schedule fun for current process. Using your system call you can read this value. Here is a snippet of the schedule function of pintos,
static void
schedule (void)
{
struct thread *cur = running_thread ();
struct thread *next = next_thread_to_run ();
struct thread *prev = NULL;
ASSERT (intr_get_level () == INTR_OFF);
ASSERT (cur->status != THREAD_RUNNING);
ASSERT (is_thread (next));<br/>
if (cur != next)
prev = switch_threads (cur, next); <== here you can update count of "cur"
thread_schedule_tail (prev);
}

Answer 4

Total number of context switches
cat /proc/PID/sched|grep nr_switches
Voluntary context switches
cat /proc/PID/sched | grep nr_voluntary_switches
Involuntary context switches
cat /proc/PID/sched|grep nr_involuntary_switches
where PID is process ID of the process you wish to monitor.
However if you want to get these statistics by patching (creating a hook) linux source, the code related to scheduling is present in
kernel/sched/
folder of the source tree.
In particular
kernel/sched/core.c contains the schedule() function, which is the code of linux scheduler.
The code of CFS (completely fair scheduler), which is one of the several schedulers present in Linux, and is most commonly used is present in
/kernel/sched/fair.c
scheduler() is executed when ever TIF_NEED_RESCHED flag is set, so find out from which all places this flag is being set (use cscope on linux source) which will give you an insight intsight into the types of context switches occurring for a process.