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

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Date:	Tue, 18 Jun 2013 11:11:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, jolsa@...hat.com,
	vincent.weaver@...ne.edu, ak@...ux.intel.com
Subject: Re: [PATCH] perf,x86: Fix shared registers mutual exclusion bug

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.

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

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