Messages in this thread | | | Date | Thu, 23 Oct 2014 10:13:12 +0200 | From | Jiri Olsa <> | Subject | Re: [PATCH v2 06/12] perf/x86: implement cross-HT corruption bug workaround |
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On Thu, Oct 23, 2014 at 10:01:08AM +0200, Stephane Eranian wrote: > On Thu, Oct 23, 2014 at 9:19 AM, Jiri Olsa <jolsa@redhat.com> wrote: > > On Wed, Oct 22, 2014 at 02:31:51PM +0200, Jiri Olsa wrote: > >> On Thu, Oct 09, 2014 at 06:34:40PM +0200, Stephane Eranian wrote: > >> > From: Maria Dimakopoulou <maria.n.dimakopoulou@gmail.com>
SNIP
> >> > + 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 we want to check sibling counter, shouldn't we check xlo->state[i] instead? like > >> > >> if (xlo->state[i] == INTEL_EXCL_EXCLUSIVE) > >> __clear_bit(i, cx->idxmsk); > >> > >> > >> and also in condition below? > > > > any comment on this? I'm curious, because it'd enlighten me > > on how this is supposed to work ;-) > > > > I dont understand why you update the sibling's counter state instead > > of the current cpuc->excl_thread_id HT, like in intel_commit_scheduling > > while you hold lock for the current HT state > > > > could you please comment, I must be missing something > > > Yes, it is a bit confusing. It comes down that what the state represents. > Let me explain. > > In get_constraints(), you compute the bitmask of possible counters by looking > at your own state in states[tid] (xl). > > In commit_constraint(), the scheduler has picked counters and you need to commit > the changes. But you don't update your state, you update your > sibling's state. Why? > Because of the bug, what you use influences what the sibling can measure. So you > update the sibling's state to reflect its new constraint. When the > sibling calls get_constraint() > it will harvest the new constraints automatically. > > Example: > > HT0 wants to program a MEM (corrupting) event, it gathers its > constraints from get_constraints(). > The mask is, let's say, 0x3. The scheduler picks counter0. Then, in > commit_constraints(), you need > to mark *HT1*'s counter0 in exclusive mode, i.e., it cannot be used > anymore on that thread.
ah, I translated the INTEL_EXCL_EXCLUSIVE state as "here's the exclusive event, sibling HT is forbiden to schedule in this counter (slot)"
cool, thanks jirka
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