linux-kernel - Re: [PATCH][RFC] Adding information of counts processes acquired how many spinlocks to schedstat

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Date:	Wed, 1 Jul 2009 15:45:03 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Hitoshi Mitake <mitake@....info.waseda.ac.jp>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>, linux-kernel@...r.kernel.org
Subject: Re: [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@....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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