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SubjectRe: [PATCH][RFC] Adding information of counts processes acquired how many spinlocks to schedstat
On Wed, Jul 01, 2009 at 09:31:39AM +0200, Ingo Molnar wrote:
>
> * Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp> wrote:
>
> > Hi,
> >
> > I wrote a test patch which add information of counts processes
> > acquired how many spinlocks to schedstat. After applied this
> > patch, /proc/<PID>/sched will change like this,
> >
> > init (1, #threads: 1)
> > ---------------------------------------------------------
> > se.exec_start : 482130.851458
> > se.vruntime : 26883.107980
> > se.sum_exec_runtime : 2316.651816
> > se.avg_overlap : 0.480053
> > se.avg_wakeup : 14.999993
> > ....
> > se.nr_wakeups_passive : 1
> > se.nr_wakeups_idle : 0
> > se.nr_acquired_spinlock : 74483
> > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
> Looks potentially useful - but it would be nice and go one step
> further and add lock acquire stats as a software-counter.
>
> Perfcounters is a feature of the latest upstream kernel, there's a
> (still very small) Wiki page about it at:
>
> http://perf.wiki.kernel.org
>
> With perfcounters we can instrument various software properties of
> the kernel as well, for example the number of page-faults in the
> system per second:
>
> $ perf stat -a -e page-faults sleep 1
>
> Performance counter stats for 'sleep 1':
>
> 294387 page-faults
>
> 1.022318527 seconds time elapsed
>
> Now, it would be nice to have a lock-acquire software-counter as
> well, which would output things like:
>
> $ perf stat -a -e lock-acquires sleep 1
>
> Performance counter stats for 'sleep 1':
>
> 294387 lock-acquires
>
> 1.022318527 seconds time elapsed
>
> Furthermore, beyond plain counts, doing this would also allow the
> profiling of lock acquire places: perf record -e lock-acquires and
> perf report would work fine.
>
> It is really easy to add a new sw counter, check how it is done for
> the pagefault counter(s), see the uses of PERF_COUNT_SW_PAGE_FAULTS
> in the following files:
>
> $ git grep -l PERF_COUNT_SW_PAGE_FAULTS
>
> arch/powerpc/mm/fault.c
> arch/x86/mm/fault.c
> include/linux/perf_counter.h
> kernel/perf_counter.c
> tools/perf/builtin-stat.c
> tools/perf/design.txt


Indeed, the raw number of lock acquired may be useful for a perf profiling
especially in the case of profile comparison.

But IMHO, this information is too much orphan and lonesome.
We would gain a lot if this information is provided per lock. Another
useful info would be the rate of the time spent in a contended state
for a given lock.

Which makes me think it may be better to use the existing ftrace lock
events as softwares counters for that, which takes into account the
following events:

- lock_acquire
- lock_release
- lock_contended
- lock_acquired

And these events are per lock.

Now the missing piece is the sampling count for events...



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