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SubjectRe: perf_events: questions about cpu_has_ht_siblings() and offcore support
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On Fri, Apr 22, 2011 at 5:03 PM, Lin Ming <ming.m.lin@intel.com> wrote:
> On Fri, 2011-04-22 at 22:41 +0800, Stephane Eranian wrote:
>> On Fri, Apr 22, 2011 at 4:31 PM, Lin Ming <ming.m.lin@intel.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@intel.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?
>>
>> Is you have not extra_regs, i.e., regs that are shared, then why would
>> you need the percore allocation?
>
> But "extra_regs" does not imply they are regs that are shared.
> It only means some events need to set extra registers to work.
>
Do you have example of such register that would not require the
extra mutual exclusion either between HT threads or between
events on the same PMU?



>>
>>
>> >
>> >>
>> >> 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:
>> >
>> >        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?
>> >
>> I think when HT is off, you will never execute the if statement, because
>> no core_id will ever match another.
>
> The "if" statement is not executed so the per-core structs allocated in
> intel_pmu_cpu_prepare is not freed.
>
> This is the intended behavior since we don't have a way to check if HT
> is off.
>
>>
>> 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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