linux-kernel - Re: [PATCH v3 6/9] perf, x86: handle multiple records in PEBS buffer

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Date:	Fri, 25 Jul 2014 18:40:41 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	"Yan, Zheng" <zheng.z.yan@...el.com>, linux-kernel@...r.kernel.org,
	mingo@...nel.org, acme@...radead.org, eranian@...gle.com
Subject: Re: [PATCH v3 6/9] perf, x86: handle multiple records in PEBS
 buffer

> Suppose two pebs events, one has exclude_kernel set. It overflows,
> before entering the kernel, the other event generates PEBS records from
> inside the kernel with both events marked in the overflow field.
> 
> And only once we leave the kernel can the exclude_kernel event tick
> again and trigger the assist, finalyl clearing the bit.
> 
> If you were to report the records to both events, one would get a lot of
> kernel info he was not entitled to.

Ok that case can be filtered in software. Shouldn't be too difficult.
Perhaps just using ip

	if (event->attr.exclude_kernel && pebs->ip >= __PAGE_OFFSET)
	    skip;
	if (event->attr.exclude_user && pebs->ip < __PAGE_OFFSET)
	    skip;

This would also help with the existing skid.

Any other concerns?

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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