linux-kernel - Re: [PATCH V2 1/6] perf,core: allow invalid context events to be part of sw/hw groups

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Message-ID: <20150416164356.GD19775@leverpostej>
Date:	Thu, 16 Apr 2015 17:43:57 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Kan Liang <kan.liang@...el.com>
Cc:	"acme@...nel.org" <acme@...nel.org>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"eranian@...gle.com" <eranian@...gle.com>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 1/6] perf,core: allow invalid context events to be
 part of sw/hw groups

> Even if you ignore the fundamental semantics of groups, there are other
> problems with allowing shared contexts:
> 
> * The *_txn functions only get called on the group leader's PMU. If your
>   system PMU has these functions, they are not called.
> 
> * Event rotation is per ctx, but now you could have some events in a CPU
>   PMU's context, and some in the uncore PMU's context. So those can race
>   with each other.
> 
> * Throttling is also per-context. So those can race with each other too.

There's also a break down of behaviour: events in the uncore context
will get migrated to another CPU in the event of a hot unplug, while
events that are grouped with CPU events (and hence live in the CPU
context) will be destroyed.

Mark.
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