Re: [PATCH 5/9] perf/x86: implement cross-HT corruption bug workaround

[Peter Zijlstra]

On Thu, Jun 05, 2014 at 04:15:17PM +0200, Stephane Eranian wrote:
> On Thu, Jun 5, 2014 at 4:04 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Wed, Jun 04, 2014 at 11:34:14PM +0200, Stephane Eranian wrote:
> >> +
> >> +     /*
> >> +      * Modify static constraint with current dynamic
> >> +      * state of thread
> >> +      *
> >> +      * EXCLUSIVE: sibling counter measuring exclusive event
> >> +      * SHARED   : sibling counter measuring non-exclusive event
> >> +      * UNUSED   : sibling counter unused
> >> +      */
> >> +     for_each_set_bit(i, cx->idxmsk, X86_PMC_IDX_MAX) {
> >> +             /*
> >> +              * exclusive event in sibling counter
> >> +              * our corresponding counter cannot be used
> >> +              * regardless of our event
> >> +              */
> >> +             if (xl->state[i] == INTEL_EXCL_EXCLUSIVE)
> >> +                     __clear_bit(i, cx->idxmsk);
> >> +             /*
> >> +              * if measuring an exclusive event, sibling
> >> +              * measuring non-exclusive, then counter cannot
> >> +              * be used
> >> +              */
> >> +             if (is_excl && xl->state[i] == INTEL_EXCL_SHARED)
> >> +                     __clear_bit(i, cx->idxmsk);
> >> +     }
> >> +
> >> +     /*
> >> +      * recompute actual bit weight for scheduling algorithm
> >> +      */
> >> +     cx->weight = hweight64(cx->idxmsk64);
> >
> > So I think we talked about this a bit; what happens if CPU0 (taking your
> > 4 core HSW-client) is first to program its counters and takes all 4 in
> > exclusive mode?
> >
> > Then there's none left for CPU4.
> >
> > Did I miss where we avoid that problem, or is that an actual issue?
> 
> Yes, this patch series does not address this problem yet. It will be
> in a second series.
> Don't have a good solution yet.

We could limit each cpu to num_counters/2 exclusive slots. That'll still
be painful with some constrained events I imagine, but in general that
should 'work' I suppose.

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