linux-kernel - Re: perf_events: questions about cpu_has_ht

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Message-Id: <1303482702.2461.40.camel@localhost>
Date:	Fri, 22 Apr 2011 22:31:42 +0800
From:	Lin Ming <ming.m.lin@...el.com>
To:	Stephane Eranian <eranian@...gle.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, 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?


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