linux-kernel - RE: [PATCH V5 4/6] perf, x86: handle multiple records in PEBS buffer

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Message-ID: <37D7C6CF3E00A74B8858931C1DB2F0770178BC4D@SHSMSX103.ccr.corp.intel.com>
Date:	Mon, 30 Mar 2015 17:43:45 +0000
From:	"Liang, Kan" <kan.liang@...el.com>
To:	Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <peterz@...radead.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"acme@...radead.org" <acme@...radead.org>,
	"eranian@...gle.com" <eranian@...gle.com>
Subject: RE: [PATCH V5 4/6] perf, x86: handle multiple records in PEBS buffer



> -----Original Message-----
> From: Andi Kleen [mailto:andi@...stfloor.org]
> Sent: Monday, March 30, 2015 1:26 PM
> To: Liang, Kan
> Cc: Peter Zijlstra; linux-kernel@...r.kernel.org; mingo@...nel.org;
> acme@...radead.org; eranian@...gle.com; andi@...stfloor.org
> Subject: Re: [PATCH V5 4/6] perf, x86: handle multiple records in PEBS
> buffer
> 
> > >  - its possible (and harmless) for the status field to contain set bits
> > >    for !PEBS events -- the proposed code is buggy here.
> > I will fix it.
> > >  - its possible to have multiple PEBS bits set even though the event
> > >    really only was for a single event -- if you count everything with
> > >    multiple PEBS bits set as a collision you're counting wrong.
> > >
> >
> > In what situation multiple PEBS bits was set for a single event?
> > Could you please give me an example?
> 
> The field in the PEBS record is just a copy of GLOBAL_STATUS (minus some
> extra bits), so if there were already bits in GLOBAL_STATUS that haven't
> been cleared yet you may see multiple bits
> 
> In a proper configuration this should be rare, as we expect the events to
> be different and run on different effective frequencies, so
> GLOBAL_STATUS should be already cleared.

The case you mentioned has already covered in the description. 
I tried both multiple different events and multiple same events.
As you said, the collision for multiple different events is rare.
The collision for multiple same events is very high, but it's not a useful
configuration

As my understanding, Peter means the multiple PEBS bits could be set
when sampling only one single event.  

Peter, could you please clarify?

Thanks,
Kan

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