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SubjectRe: perf_events: questions about cpu_has_ht_siblings() and offcore support
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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.

We had some long discussions months ago about how to check if HT is
enabled. http://marc.info/?t=129346430400004&r=1&w=2

But unfortunately, we didn't find a way to check if HT is enabled.
So I just check if HT is supported.

>
> On my systems (NHM or SNB), its value does not change
> when I enable/disable HT.
>
> Looking at Intel's AP-485 (CPUID documentation), they
> clearly say that none of the Leaf functions which report
> about HT or the number of logical cores, can be used to
> detect HT enabled or disabled. Seems those leaf functions
> are the basis for smp_num_siblings. The trick in Table-5.5
> for bit 28 with CPUID(1).EBX[23:16] used in detect_ht()
> is about HT supported and not HT enabled.
>
> HT supported means multi-core or multi-thread supported.
>
> Going back to the perf_event code, I wonder what is the
> point of using this function in intel_pmu_cpu_prepare(), then.
>
> I suspect you wanted to know whether or not HT was enabled.
> But that's not going to work. If you want that functionality, then
> I tried:
>
> +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;
> +}
>
> But that cannot be used in the CPU hotplug callback for prepare(),
> it is too early.
>
> 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.

>
> I am preparing a patch that builds on your patch and improves
> support for those events or features which require an extra
> (shared) register. They are differences between NHM/WSM
> and SNB.




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