linux-kernel - Re: [PATCH] x86, perf: Use INST_RETIRED.PREC

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Message-ID: <20151020222844.GK3533@two.firstfloor.org>
Date:	Wed, 21 Oct 2015 00:28:44 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86, perf: Use INST_RETIRED.PREC_DIST for cycles:pp on
 Skylake

> Typically on workstations you do not, because there's only a single
> user, but on servers it might be more common.  The thing I expect to be
> most common is having both a CPU wide and a per task cycle counter
> enabled.

With multiple users they would sample only their own processes.
That works fine. The only thing that needs multiplexing is overlapping
sampling.

perf top also works fine because it doesn't use PEBS by default.

> And you having killed the flag bits for PEBS events precludes people
> from using this manually, right?  I think we want to exempt .inv=1
> .cmask=16 from that general rule on general utility value.

I don't want to do this for every event. It has caused problems
in the past. There's also no reason to use the other events.

> We could maybe abuse .precise_ip = 3 for this?

I implemented this now, even though it's ugly.

-Andi
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