linux-kernel - Re: perf_events: questions about cpu_has_ht

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Message-Id: <1303486219.2461.61.camel@localhost>
Date:	Fri, 22 Apr 2011 23:30:19 +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 23:05 +0800, Stephane Eranian wrote:
> On Fri, Apr 22, 2011 at 5:03 PM, Lin Ming <ming.m.lin@...el.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@...el.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@...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?
> >>
> >> 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?

No.

I was thinking the case that "extra_regs" may be per-thread, instead of
pef-core.

So if there are "extra_regs" or not does not connected directly with if
locking is needed or not.

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