linux-kernel - Re: [PATCH RFC] x86: avoid atomic operation in test_and_set_bit

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Date:	Thu, 24 Mar 2011 21:54:22 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Jack Steiner <steiner@....com>, Jan Beulich <JBeulich@...ell.com>,
	Borislav Petkov <bp@...64.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...nel.dk>,
	"x86@...nel.org" <x86@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>, tee@....com,
	Nikanth Karthikesan <knikanth@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH RFC] x86: avoid atomic operation in test_and_set_bit_lock
 if possible


* Eric Dumazet <eric.dumazet@...il.com> wrote:

> Le jeudi 24 mars 2011 à 21:00 +0100, Ingo Molnar a écrit :
> 
> > One good way to see cache bounces is to run a misses/accesses ratio profile:
> > 
> >    perf top -e cache-misses -e cache-references --count-filter 10
> > 
> 
> Oh well , something must be broken...
> 
> "perf top" is working here, but if I use any "-e ...." argument, it
> fails :
> 
> # perf top -e cache-misses
> 
>   Error: sys_perf_event_open() syscall returned with 2 (No such file or directory).
>   /bin/dmesg may provide additional information.
> 
>   Fatal: No CONFIG_PERF_EVENTS=y kernel support configured?
> 
> # grep PERF_EVENTS .config
> CONFIG_HAVE_PERF_EVENTS=y
> CONFIG_PERF_EVENTS=y
> CONFIG_HAVE_PERF_EVENTS_NMI=y
> 
> Nothing in dmesg...
> 
> uname -a
> Linux ed001 2.6.38-08165-g2712750-dirty #517 SMP Thu Mar 24 21:31:00 CET 2011 i686 i686 i386 GNU/Linux

Does it work better if you run it as root?

To easiest way to debug as user is to do:

  echo -1 > /proc/sys/kernel/perf_event_paranoid 

and to put this into /etc/sysctl.conf:

  kernel.perf_event_paranoid = -1

Thanks,

	Ingo
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