linux-kernel - Re: Possible race between CPU hotplug and perf_pmu_migrate

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Message-ID: <20140905195439.GU4783@worktop.ger.corp.intel.com>
Date:	Fri, 5 Sep 2014 21:54:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Mark Rutland <mark.rutland@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Yan Zheng <zheng.z.yan@...el.com>,
	Stephane Eranian <eranian@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Vince Weaver <vincent.weaver@...ne.edu>
Subject: Re: Possible race between CPU hotplug and perf_pmu_migrate_context

On Fri, Sep 05, 2014 at 10:31:49AM -0700, Linus Torvalds wrote:
> So quite frankly, the whole perf_pmu_migrate_context() thing looks
> completely and fundamentally broken.

Yes, agreed. We can go play very nasty games, but fundamentally agreed.

> Or even just say: "if somebody takes down a CPU with existing uncore
> events, those events are now effectively dead". Don't try to migrate
> them, don't try to create new events on another CPU, just let it go.
> The CPU is down, the events are inactive.
> 
> Keep it simple. Not the current completely broken thing that clearly
> doesn't honor the actual real rules for "event->ctx". Because the
> current rules are effectively that event->ctx is a constant over the
> whole lifetime of the event. Agreed?

I like this best, I've never really been a supporter of hotplug games. 
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