Re: [PATCH -tip] x86,perf: P4 PMU -- protect sensible procedures from preemption

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Thursday, May 6, 2010, Ingo Molnar <mingo@elte.hu> wrote:
>
> * Cyrill Gorcunov <gorcunov@gmail.com> wrote:
>
>> On Thursday, May 6, 2010, Ingo Molnar <mingo@elte.hu> wrote:
>> >
>> > * Cyrill Gorcunov <gorcunov@gmail.com> wrote:
>> >
>> >> On Wed, May 05, 2010 at 06:57:34PM +0200, Frederic Weisbecker wrote:
>> >> ...
>> >> > > @@ -741,7 +743,7 @@ static int p4_pmu_schedule_events(struct
>> >> > > ?{
>> >> > > ? unsigned long used_mask[BITS_TO_LONGS(X86_PMC_IDX_MAX)];
>> >> > > ? unsigned long escr_mask[BITS_TO_LONGS(ARCH_P4_TOTAL_ESCR)];
>> >> > > - int cpu = raw_smp_processor_id();
>> >> > > + int cpu = get_cpu();
>> >> > > ? struct hw_perf_event *hwc;
>> >> > > ? struct p4_event_bind *bind;
>> >> > > ? unsigned int i, thread, num;
>> >> > > @@ -777,6 +779,7 @@ reserve:
>> >> > > ? }
>> >> > >
>> >> > > ?done:
>> >> > > + put_cpu();
>> >> > > ? return num ? -ENOSPC : 0;
>> >> > > ?}
>> >> >
>> >> > That's no big deal. But I think the schedule_events() is called on
>> >> > pmu::enable() time, when preemption is already disabled.
>> >> >
>> >>
>> >> We'll be on a safe side using get/put_cpu here (ie in case
>> >> if something get changed one day).
>> >
>> > hm, when 'something gets changed one day' we'll see a warning when using
>> > unsafe primitives.
>> >
>> > So if preemption is always off here we really should not add extra runtime
>> > overhead via get_cpu()/put_cpu().
>> >
>> > So wouldnt it be better (and faster) to disable preemption in
>> > hw_perf_event_init(), which seems to be the bit missing?
>> >
>> >  ? ? ? ?Ingo
>> >
>>
>> the thing are that p4 is only snippet here which is sensible to preemtion,
>> and hw_perf_event_init is executing with preemtion off (but i could miss the
>> details here, dont have code under my hands at moment, so PeterZ help is
>> needed ;) but more important reason why i've saved get/put here is that
>> otherwise i would not have rights to put tested-by tag, since it would not
>> be the patch Steven has tested. We could make a patch on top of this one, or
>> we could drop this one, make new with explicit preemt off in caller and use
>> smp_processor_id in p4 schedule routine. What is preferred?
>
> We want the one with the least runtime overhead. These are instrumentation
> routines, so we want to optimize them as much as possible.
>
> Thanks,
>
>         Ingo
>

ok, Ingo, dont apply this patch then for a while.