linux-kernel - Re: [LKP] [lkp] [xfs] 68a9f5e700: aim7.jobs-per-min -13.6% regression

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Message-ID: <20160817162451.GC24652@twins.programming.kicks-ass.net>
Date:	Wed, 17 Aug 2016 18:24:51 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Chinner <david@...morbit.com>, Tejun Heo <tj@...nel.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Christoph Hellwig <hch@....de>,
	"Huang, Ying" <ying.huang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Bob Peterson <rpeterso@...hat.com>, LKP <lkp@...org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>
Subject: Re: [LKP] [lkp] [xfs] 68a9f5e700: aim7.jobs-per-min -13.6% regression

On Mon, Aug 15, 2016 at 07:03:00AM +0200, Ingo Molnar wrote:
> 
> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > Make sure you actually use "perf record -e cycles:pp" or something
> > that uses PEBS to get real profiles using CPU performance counters.
> 
> Btw., 'perf record -e cycles:pp' is the default now for modern versions
> of perf tooling (on most x86 systems) - if you do 'perf record' it will
> just use the most precise profiling mode available on that particular
> CPU model.
> 
> If unsure you can check the event that was used, via:
> 
>   triton:~> perf report --stdio 2>&1 | grep '# Samples'
>   # Samples: 27K of event 'cycles:pp'

Problem here is that Dave is using a KVM thingy. Getting hardware
counters in a guest is somewhat tricky but doable, but PEBS does not
virtualize.