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

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Date:	Thu, 20 Jun 2013 14:07:43 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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


* Stephane Eranian <eranian@...gle.com> wrote:

> 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.

That happened yesterday, so latest -tip should be a good base to work on.

Thanks,

	Ingo
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