Re: perf_events: questions about cpu_has_ht_siblings() and offcore support

[Stephane Eranian]

On Fri, Apr 22, 2011 at 4:31 PM, Lin Ming <ming.m.lin@xxxxxxxxx> 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@xxxxxxxxx> 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)
>> >> {
>> >> Â Â Â Âbool has_siblings = false;
>> >> #ifdef CONFIG_SMP
>> >> Â Â Â Âhas_siblings = cpu_has_ht && smp_num_siblings > 1;
>> >> #endif
>> >> Â Â Â Âreturn 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)
>> >> +{
>> >> + Â Â Â bool has_ht = false;
>> >> +#ifdef CONFIG_SMP
>> >> + Â Â Â int w;
>> >> + Â Â Â w = cpumask_weight(cpu_sibling_mask(smp_processor_id()));
>> >> + Â Â Â has_ht = cpu_has_ht && w > 1;
>> >> +#endif
>> >> + Â Â Â 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?

Is you have not extra_regs, i.e., regs that are shared, then why would
you need the percore allocation?


>
>>
>> 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.
>
Yes. You won't need the locking, though.

>>
>> 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:
>
> Â Â Â Âfor_each_cpu(i, topology_thread_cpumask(cpu)) {
> Â Â Â Â Â Â Â Âstruct intel_percore *pc = per_cpu(cpu_hw_events, i).per_core;
>
> Â Â Â Â Â Â Â Âif (pc && pc->core_id == core_id) {
> Â Â Â Â Â Â Â Â Â Â Â Âkfree(cpuc->per_core);
> Â Â Â Â Â Â Â Â Â Â Â Âcpuc->per_core = pc;
> Â Â Â Â Â Â Â Â Â Â Â Âbreak;
> Â Â Â Â Â Â Â Â}
> Â Â Â Â}
>
> Or do you see other potential problem?
>
I think when HT is off, you will never execute the if statement, because
no core_id will ever match another.

Another thing that struck me when locking at the hotplug code for
per-core is the lack of locking. I assume that's because hotplug
cpu is inherently serialized. You cannot have a CPU going offline
and one going online at the same time. is that right? Otherwise
I wonder if you could simply do per_core->refcnt++ vs.
per_core->refcnt--
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