Messages in this thread | | | Subject | Re: perf_events: questions about cpu_has_ht_siblings() and offcore support | From | Lin Ming <> | Date | Fri, 22 Apr 2011 23:03:17 +0800 |
| |
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.
> > > > > >> > >> 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--
| |