Messages in this thread | | | Date | Fri, 22 Apr 2011 17:05:37 +0200 | Subject | Re: perf_events: questions about cpu_has_ht_siblings() and offcore support | From | Stephane Eranian <> |
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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-- > > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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