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

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Date:	Mon, 28 Jul 2014 08:54:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"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

On Fri, Jul 25, 2014 at 06:40:41PM +0200, Andi Kleen wrote:
> > 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?

Yeah, why fuck about and do ugly hacks when you can actually do it
right? That way you're sure you've not forgotten anything.

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