linux-kernel - Re: [PATCH 4/9] perf/x86: add cross-HT counter exclusion infrastructure

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Date:	Tue, 10 Jun 2014 14:26:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Jiri Olsa <jolsa@...hat.com>,
	"Yan, Zheng" <zheng.z.yan@...el.com>,
	Maria Dimakopoulou <maria.n.dimakopoulou@...il.com>
Subject: Re: [PATCH 4/9] perf/x86: add cross-HT counter exclusion
 infrastructure

On Tue, Jun 10, 2014 at 01:53:45PM +0200, Stephane Eranian wrote:
> On Thu, Jun 5, 2014 at 10:29 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Wed, Jun 04, 2014 at 11:34:13PM +0200, Stephane Eranian wrote:
> >> @@ -2020,12 +2050,29 @@ static void intel_pmu_cpu_starting(int cpu)
> >>
> >>       if (x86_pmu.lbr_sel_map)
> >>               cpuc->lbr_sel = &cpuc->shared_regs->regs[EXTRA_REG_LBR];
> >> +
> >> +     if (x86_pmu.flags & PMU_FL_EXCL_CNTRS) {
> >> +             for_each_cpu(i, topology_thread_cpumask(cpu)) {
> >> +                     struct intel_excl_cntrs *c;
> >> +
> >> +                     c = per_cpu(cpu_hw_events, i).excl_cntrs;
> >> +                     if (c && c->core_id == core_id) {
> >> +                             cpuc->kfree_on_online[1] = cpuc->excl_cntrs;
> >> +                             cpuc->excl_cntrs = c;
> >> +                             cpuc->excl_thread_id = 1;
> >> +                             break;
> >> +                     }
> >> +             }
> >> +             cpuc->excl_cntrs->core_id = core_id;
> >> +             cpuc->excl_cntrs->refcnt++;
> >> +     }
> >>  }
> >
> > This hard assumes theres only ever 2 threads, which is true and I
> > suppose more in arch/x86 will come apart the moment Intel makes a chip
> > with more, still, do we have topology_thread_id() or so to cure this?
> 
> I assume your comment is relative to kfree_on_online[].
> This code is specific to the HT bug, so yes, it assumes 2 threads and that
> only one entry of the two excl_cntrs structs needs to be freed.
> Doing otherwise, would require a list and will never be used to its full
> potential.

That and ->excl_thread_id = 1, I was thinking that if we'd somehow have
4 threads, some of them need to have = [23] in there.



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