Re: [PATCH v2 02/11] perf/x86: Improve HT workaround GP counter constraint

[Stephane Eranian]

On Tue, May 26, 2015 at 3:15 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> Please trim your email.
>
> On Tue, May 26, 2015 at 02:37:52AM -0700, Stephane Eranian wrote:
>> > @@ -822,8 +830,24 @@ int x86_schedule_events(struct cpu_hw_ev
>> >
>> >         /* slow path */
>> >         if (i != n) {
>> > +               int gpmax = x86_pmu.num_counters;
>> > +
>> > +               /*
>> > +                * Do not allow scheduling of more than half the available
>> > +                * generic counters.
>> > +                *
>> > +                * This helps avoid counter starvation of sibling thread by
>> > +                * ensuring at most half the counters cannot be in exclusive
>> > +                * mode. There is no designated counters for the limits. Any
>> > +                * N/2 counters can be used. This helps with events with
>> > +                * specific counter constraints.
>> > +                */
>> > +               if (is_ht_workaround_enabled() && !cpuc->is_fake &&
>> > +                   READ_ONCE(cpuc->excl_cntrs->exclusive_present))
>> > +                       gpmax /= 2;
>> > +
>> What I don't like about this part is that this is a hack to work around a bug
>> on some limited Intel CPUs and yet it is in the middle of generic x86 code.
>> I understand it will be inoperative on AMD PMU and is not used by Intel
>> uncore. On KNC or P6, you will not have is_ht_workaround_enabled().
>> Could this be made a x86_pmu callback? x86_pmu.counter_limit()?
>
> It'll be slower though. You get an indirect function call in there.
>
> But sure we can clean that up later if you like; there's other things
> needing to be fixed here first.
>
> I'm going to overhaul the whole get/put constraints stuff first.

Ok, I think it would be good to balance to number of get/put. It would
avoid the confusion. Is that what you are thinking about?
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/