On 04/11, Masami Hiramatsu wrote:
>
> (2013/04/10 23:58), Oleg Nesterov wrote:
>
> > And... Cough, another question ;) To simplify, lets discuss
> > kprobe_perf_func()
> > only. Suppose that a task hits the kprobe but this task/cpu doesn't have
> > a counter. Can't we avoid perf_trace_buf_prepare/submit in this case?
> > IOW, what do you think about the change below?
>
> Hmm, I'm not so sure how frequently this happens.
Suppose that you do, say, "perf record -e probe:some_func workload". Only
"workload" will have the active counter, any other task which hits the
probed some_func() will do perf_trace_buf_prepare/perf_trace_buf_submit
just to realize that nobody wants perf_swevent_event().
Simple test-case:
#include <unistd.h>
int main(void)
{
int n;
for (n = 0; n < 1000 * 1000; ++n)
getppid();
return 0;
}
Without kprobe:
# time ./ppid
real 0m0.663s
user 0m0.163s
sys 0m0.500s
Activate the probe:
# perf probe sys_getppid
# perf record -e probe:sys_getppid sleep 1000 &
[1] 546
Test it again 3 times:
# time ./ppid
Before the patch:
real 0m9.727s
user 0m0.177s
sys 0m9.547s
real 0m9.752s
user 0m0.180s
sys 0m9.573s
real 0m9.761s
user 0m0.187s
sys 0m9.573s
After the patch:
real 0m9.605s
user 0m0.163s
sys 0m9.437s
real 0m9.592s
user 0m0.167s
sys 0m9.423s
real 0m9.613s
user 0m0.183s
sys 0m9.427s
So the difference looks measurable but small, and I did the testing
under qemu so I do not really know if we can trust the numbers.
> And, is this right way to
> handle that case?
If only I was sure ;) I am asking.
And, to clarify, it is not that I think this change can really
improve the perfomance. Just I am trying to understand what I have
missed.
> If so, we can do same thing also on trace_events.
> (perf_trace_##call in include/trace/ftrace.h)
Yes, yes, this is not kprobe-specific. It seems that more users of
perf_trace_buf_submit() could be changed the same way.
Thanks,
Oleg.
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