How to understand "$location" in a Systemtap script? - linux

Question

The Systemtap script:
# Array to hold the list of drop points we find
global locations
# Note when we turn the monitor on and off
probe begin { printf("Monitoring for dropped packets\n") }
probe end { printf("Stopping dropped packet monitor\n") }
# increment a drop counter for every location we drop at
#probe kernel.trace("kfree_skb") { locations[$location] <<< 1 }
# Every 5 seconds report our drop locations
probe timer.sec(5)
{
printf("\n")
foreach (l in locations-) {
printf("%d packets dropped at location %p\n",
#count(locations[l]), l)
}
delete locations
}
and the source code of kfree_skb() is:
void kfree_skb(struct sk_buff *skb)
{
if (unlikely(!skb))
return;
if (likely(atomic_read(&skb->users) == 1))
smp_rmb();
else if (likely(!atomic_dec_and_test(&skb->users)))
return;
trace_kfree_skb(skb, __builtin_return_address(0));
__kfree_skb(skb);
}
I just want to know where is the $location from? And
what is the relationship between $location and kfree_skb()?Thank you.

Answer 1

As per the stap.1 man page:
Many types of probe points provide context variables, which are
run-time values, safely extracted from the kernel or userspace
program being probed. These are pre‐ fixed with the $
character. The CONTEXT VARIABLES section in stapprobes(3stap)
lists what is available for each type of probe point.
As per the stapprobes.3stap man page:
KERNEL TRACEPOINTS
This family of probe points hooks up to static probing
tracepoints inserted into the kernel or modules. [...]
Tracepoint probes look like: kernel.trace("name"). The
tracepoint name string, which may contain the usual wildcard
characters, is matched against the names defined by the kernel
developers in the tracepoint header files.
The handler associated with a tracepoint-based probe may read
the optional parame‐ ters specified at the macro call site.
[...] For example, the tracepoint probe kernel.trace("sched_switch")
provides the parameters $rq, $prev, and $next. [...]
The name of the tracepoint is available in $$name, and a string
of name=value pairs for all parameters of the tracepoint is
available in $$vars or $$parms.
As per the linux kernel source code:
% cd net/core
% git grep trace_kfree_skb
dev.c: [...]
drop_monitor.c: [...]
skbuff.c: [...]
% cd ../../include/trace/events
% git grep -A5 'TRACE_EVENT.*kfree_skb'
skb.h:TRACE_EVENT(kfree_skb,
skb.h-
skb.h- TP_PROTO(struct sk_buff *skb, void *location),
skb.h-
skb.h- TP_ARGS(skb, location),
skb.h-