linux-kernel - Re: [PATCH v2] perf,x86: Fix shared register mutual exclusion enforcement

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Date:	Tue, 25 Jun 2013 11:54:09 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>, vincent.weaver@...ne.edu,
	Jiri Olsa <jolsa@...hat.com>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: Re: [PATCH v2] perf,x86: Fix shared register mutual exclusion enforcement

On Mon, Jun 24, 2013 at 5:48 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Jun 24, 2013 at 10:01:26AM +0200, Stephane Eranian wrote:
>
>> You are missing the error path in schedule_events():
>>
>>  if (!assign || num) {
>>
>>                 for (i = 0; i < n; i++) {
>>                         if (x86_pmu.put_event_constraints)
>>                                 x86_pmu.put_event_constraints(cpuc,
>> cpuc->event_list[i]);
>>                 }
>>
>>         }
>>
>> That one wipes out on get() even on events that were correctly
>> schedule in the previous
>> invocation. So here group2 fails, but it should not release the
>> constraints from group1.
>
> What I was saying:
>
>  schedule(group1)
>    get_event_constraints() +1
>    no error path, no puts
>
>  schedule(group2)
>    get_event_constraints() +1
>    *fail*
>      put_event_constraints() -1
>
> This leaves the constraints of group1 with a net +1 'ref' count and thus
> if we were to treat the get/put as such, the put wouldn't be the last
> and thus shouldn't release resources.
>
>> > Only once these events pass through x86_pmu_del() will they get a final
>> > put and the 'ref' count will drop to 0.
>> >
>> > Now the problem seems to be the get/put things don't actually count
>> > properly.
>> >
>> > However, if we look at __intel_shared_reg_{get,put}_constraints() there
>> > is a refcount in there; namely era->ref; however we don't appear to
>> > clear reg->alloc based on it.
>> >
>> The era->ref is not used to ref count the number of successful attempts
>> at scheduling. It is used to count the number of CPU sharing the resource.
>> So it goes from 0, 1, to 2. You can invoke schedule_events() many more
>> times. The reg->alloc is a bypass, to avoid checking the shared reg
>> again and again if it succeeded once.
>
> Oh right, I knew I was missing something..  :/
>
>> For a while I thought I could leverage the era->ref to account the get/put.
>> But it does not work. Because the of the put().
>
> Crud, right you are.
>
> Also, I don't think we could even use them as I outlined; suppose it
> would have worked; then we'd have:
>
>   schedule(group1)
>     get_event_constraints() +1
>
>   schedule(group2)
>     get_event_constraints() +1
>
> And we'd be stuck with a ref of 2, the put at x86_pmu_del() would never
> be sufficient to drop them back to 0 again.
>
Yes, imagine we add 3 groups:

    schedule(grp1)  grp1=1

    schedule(grp2) grp1=2, grp2 =1

   schedule(grp3) -> *failed*, put, grp1=2, grp2=1, grp3=0

And then in the del side, we'd have no way of knowing that to get
the current assignment we incrementally scheduled twice, and thus
we have to do grp1 -=2 and not just -=1.


> A well, your patch does indeed make it work so I'll grab that.

Thanks,
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