linux-kernel - Re: [PATCH] perf,x86: Fix shared registers mutual exclusion bug

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Message-ID: <CABPqkBTtnX3KNyVXzXwL82EADhs14v-LCuoxLgGnXPTP_FaSFQ@mail.gmail.com>
Date:	Tue, 18 Jun 2013 20:43:08 +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>, Jiri Olsa <jolsa@...hat.com>,
	vincent.weaver@...ne.edu, "ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: Re: [PATCH] perf,x86: Fix shared registers mutual exclusion bug

On Tue, Jun 18, 2013 at 11:11 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, Jun 05, 2013 at 04:43:46PM +0200, Stephane Eranian wrote:
>>
>> This patch fixes a problem with the shared registers mutual
>> exclusion code and incremental event scheduling by the
>> generic perf_event code.
>>
>> There was a bug whereby the mutual exclusion on the shared
>> registers was not enforced because of incremental scheduling
>> abort due to event constraints.
>>
>> Example on Nehalem:
>> group1= ref-cycles,OFFCORE_RESPONSE_0:PF_RFO
>> group2= ref-cycles
>>
>> The ref-cycles event can only be measured by 1 counter. Yet, there
>> are 2 instances here. The first group can be scheduled and is committed.
>> Then, the generic code tries to schedule group2 and this fails (because
>> there is no more counter to support the 2nd instance of ref-cycles).
>>
>> But in x86_schedule_events() error path, put_event_contraints() is invoked
>> on ALL the events and not just the ones that just failed. That causes the
>> "lock" on the shared offcore_response MSR to be released. Yet the first group
>> is actually scheduled and is exposed to reprogramming of that shared msr by
>> the sibling HT thread (when they are shared by HT threads). In other words,
>> there is no guarantee on what is measured for the offcore_response event.
>>
>> This patch fixes the problem by tagging committed events with the
>> PERF_X86_EVENT_COMMITTED tag. In the error path of x86_schedule_events(),
>> only the events NOT tagged have their constraint released. The tag
>> is eventually removed when the event in descheduled.
>>
>> Example was given with offcore_response but also applies to LBR_SELECT
>> and LDLAT shared registers.
>>
>> Signed-off-by: Stephane Eranian <eranian@...gle.com>
>
> I'm getting conflicts against other patches -- most notably I think the
> contraints stack opt from Andrew Hunter.
>
Yes, that would not surprise me. I wrote this patch without assuming
Andrew's patch would be there. But we need to add it. Then we can fix
the shared_regs patch.

> I'll try and get Ingo to finally pick up my queued patches so we can
> rebase.

Ok, thanks.

>
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