linux-kernel - Re: perf_events: questions about cpu_has_ht

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Date:	Fri, 22 Apr 2011 15:46:09 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Lin Ming <ming.m.lin@...el.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, 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
>> 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.

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.

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.
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