linux-kernel - Re: [PATCH 7/7] perf: kill perf_event

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Message-ID: <20140227123035.GA8647@e106331-lin.cambridge.arm.com>
Date:	Thu, 27 Feb 2014 12:30:35 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Will Deacon <Will.Deacon@....com>,
	Dave P Martin <Dave.Martin@....com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 7/7] perf: kill perf_event_context::pmu

On Thu, Feb 27, 2014 at 11:51:43AM +0000, Peter Zijlstra wrote:
> On Thu, Feb 27, 2014 at 11:48:05AM +0000, Mark Rutland wrote:
> > On Tue, Feb 25, 2014 at 11:31:00AM +0000, Peter Zijlstra wrote:
> > > On Tue, Feb 11, 2014 at 05:56:51PM +0000, Mark Rutland wrote:
> > > > Another option would be to have a context per-pmu. Each context's pmu
> > > > pointer would be valid, and (other than the case of software events) it
> > > > doesn't make sense to place events from disparate PMUs into the same
> > > > group anyway. Then you don't need a fixed sized pmu list in the context
> > > > or some arcane list structs.
> > > 
> > > No it does make sense; for example on hardware that doesn't have a PMI
> > > you can create a software event + hardware event group and have the
> > > software interrupt read the hardware counter and still get 'some'
> > > sampling.
> > 
> > Sure, I called out software events as an exception above.
> 
> Oh sorry missed that.
> 
> > Does it ever make sense to group two hardware events for disparate
> > hardware PMUs?
> 
> No, and I think we disallow that. We only explicitly allow software
> events/groups to move to !software context.

As you say, other than the sw leader, hw follower case we currently
check that event->ctx is equivalent. This works for homogeneous PMUs, as
we only ever have one PMU in any task's
perf_event_ctxp[perf_hw_context].

In the heterogeneous PMU case multiple CPU PMUs can share the
perf_hw_context in a task, so the check for context equivalence is not
sufficient.

I can hide the check in the architecture backend, or we could try to
make the core code be more picky with something like the below.

---->8----
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 56003c6..a4b6f80 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7135,6 +7135,8 @@ SYSCALL_DEFINE5(perf_event_open,
                        if (group_leader->ctx->type != ctx->type)
                                goto err_context;
                } else {
+                       if (group_leader->pmu != pmu)
+                               goto err_context;
                        if (group_leader->ctx != ctx)
                                goto err_context;
                }
---->8----

Cheers,
Mark.
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