MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Message-ID: <18901.11489.765420.408337@cargo.ozlabs.ibm.com>
Date: Fri, 3 Apr 2009 08:23:45 +1100
From: Paul Mackerras <paulus@samba.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Corey Ashford <cjashfor@linux.vnet.ibm.com>,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/6] RFC perf_counter: singleshot support
In-Reply-To: <20090402122616.GB24618@elte.hu>
References: <20090402091158.291810516@chello.nl>
	<20090402091319.257773792@chello.nl>
	<20090402105151.GB10828@elte.hu>
	<1238672893.8530.5909.camel@twins>
	<20090402122616.GB24618@elte.hu>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1240
Lines: 33

Ingo Molnar writes:

> Sure. One sidenote - the precision of sw clocks has dropped a bit 
> lately:
> 
> aldebaran:~/linux/linux/Documentation/perf_counter> ./perfstat -e 
> 1:0 -e 1:0 -e 1:0 -e 1:0 -e 1:0 sleep 1
> 
>  Performance counter stats for 'sleep':
> 
>        0.762664  cpu clock ticks      (msecs)
>        0.761440  cpu clock ticks      (msecs)
>        0.760977  cpu clock ticks      (msecs)
>        0.760587  cpu clock ticks      (msecs)
>        0.760287  cpu clock ticks      (msecs)
> 
>  Wall-clock time elapsed:  1003.139373 msecs
> 
> See that slight but noticeable skew? This used to work fine and we 
> had the exact same value everywhere. Can we fix that while still 
> keeping the code nice?

I suggest basing the software clock on counter->ctx->time_now, and
make get_context_time use cpu_clock() always.  That way we will only
call cpu_clock() once even if we have multiple cpu clock counters, and
that will eliminate the skew as well as being more efficient.

Paul.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/