linux-kernel - Re: perf_events: questions about cpu_has_ht

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Date:	Fri, 22 Apr 2011 16:53:22 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Lin Ming <ming.m.lin@...el.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	"Kleen, Andi" <andi.kleen@...el.com>
Subject: Re: perf_events: questions about cpu_has_ht_siblings() and offcore support

On Fri, Apr 22, 2011 at 4:31 PM, Lin Ming <ming.m.lin@...el.com> wrote:
> On Fri, 2011-04-22 at 21:46 +0800, Stephane Eranian wrote:
>> On Fri, Apr 22, 2011 at 3:26 PM, Lin Ming <ming.m.lin@...el.com> wrote:
>> > On Fri, 2011-04-22 at 20:59 +0800, Stephane Eranian wrote:
>> >> Lin,
>> >>
>> >> In arch/x86/include/asm/smp.h, you added:
>> >>
>> >> static inline bool cpu_has_ht_siblings(void)
>> >> {
>> >>        bool has_siblings = false;
>> >> #ifdef CONFIG_SMP
>> >>        has_siblings = cpu_has_ht && smp_num_siblings > 1;
>> >> #endif
>> >>        return has_siblings;
>> >> }
>> >>
>> >> I am wondering about the goal of this function.
>> >>
>> >> Is it supposed to return whether or not HT is enabled?
>> >>
>> >> Ht enabled != HT supported
>> >
>> > It's used to check if HT is supported.
>> >
>> Ok, that makes more sense.
>>
>> > But unfortunately, we didn't find a way to check if HT is enabled.
>> > So I just check if HT is supported.
>> >
>> >>
>> >> +static inline int is_ht_enabled(void)
>> >> +{
>> >> +       bool has_ht = false;
>> >> +#ifdef CONFIG_SMP
>> >> +       int w;
>> >> +       w = cpumask_weight(cpu_sibling_mask(smp_processor_id()));
>> >> +       has_ht = cpu_has_ht && w > 1;
>> >> +#endif
>> >> +       return has_ht;
>> >> +}
>> >>
>> >> OTOH, you need some validation even in the case HT is off. No two events
>> >> scheduled together on the same PMU can have different values for the extra
>
> I got it now.
>
>> >> reg. Thus, the fact that cpu_has_ht_siblings() is imune to HT state helps here,
>> >> but then what's the point of it?
>> >
>> > The points is to avoid the percore resource allocations(which are used
>> > to sync between HTs) if HT is not supported.
>> >
>> But if you check x86_pmu.extra_regs, that should do it as well.
>
> I don't understand here.
> Did you mean we can avoid the percore resource allocations by just
> checking x86_pmu.extra_regs? How?
>
>>
>> Suppose HT is disabled and I do:
>>
>> perf stat -e offcore_response_0:dmd_data_rd,offcore_response_0:dmnd_rfo ......
>>
>> This should still not be allowed.
>
> Ah, you are right.
> We have to always check extra_config even HT is disabled and/or
> supported.
>
>>
>> I think in this case, HT supported will cause your code to still allocate the
>> per-core struct. There will be no matching of per-core structs in starting().
>> So I suspect things work.
>
> This has no problem.
> If "no matching" found, then below if(...) statement won't be executed.
>
> intel_pmu_cpu_starting:
>
>        for_each_cpu(i, topology_thread_cpumask(cpu)) {
>                struct intel_percore *pc = per_cpu(cpu_hw_events, i).per_core;
>
>                if (pc && pc->core_id == core_id) {
>                        kfree(cpuc->per_core);
>                        cpuc->per_core = pc;
>                        break;
>                }
>        }
>
> Or do you see other potential problem?
>
The other problem I found while working on my patch is that the
current code, allows you to construct an event group which will
never be schedulable:

task -e offcore_response_0:dmd_data_rd,offcore_response_0:dmnd_rfo ....

That's because validate_group() uses a fake_cpuc which does not implement
the per-core logic. I think I understand how why you had:

intel_percore_constraints(struct cpu_hw_events *cpuc, struct perf_event *event)
{
             .....
                pc = cpuc->per_core;
                if (!pc)
                        break;
             ....
}

I have that fixed in my patch.
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