linux-kernel - Re: [PATCH V6 4/6] perf, x86: handle multiple records in PEBS buffer

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Message-ID: <20150415182836.GX23123@twins.programming.kicks-ass.net>
Date:	Wed, 15 Apr 2015 20:28:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kan Liang <kan.liang@...el.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, acme@...radead.org,
	eranian@...gle.com, andi@...stfloor.org
Subject: Re: [PATCH V6 4/6] perf, x86: handle multiple records in PEBS buffer

On Thu, Apr 09, 2015 at 12:37:44PM -0400, Kan Liang wrote:

> The only way you can get a lot of collision is when you count the same
> thing multiple times. But it is not a useful configuration.

Not entirely true; I _think_ you can be unfortunate if you measure with
a userspace only PEBS event along with either a kernel or unrestricted
PEBS event. Imagine the event triggering and setting the overflow flag
right before entering the kernel. Then all kernel side events will end
up with multiple bits set.


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