linux-kernel - Re: [PATCH for testing] Re: Decreasing stime running confuses top

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Message-ID: <20071008170023.GA31765@elte.hu>
Date:	Mon, 8 Oct 2007 19:00:23 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	Frans Pop <elendil@...net.nl>, Chuck Ebbert <cebbert@...hat.com>,
	Luca Tettamanti <kronos.it@...il.com>,
	Willy Tarreau <w@....eu>, LKML <linux-kernel@...r.kernel.org>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	"Alexander E. Patrakov" <patrakov@....usu.ru>
Subject: Re: [PATCH for testing] Re: Decreasing stime running confuses top


* Christian Borntraeger <borntraeger@...ibm.com> wrote:

> Why does it still shows numbers going backwards? I guess the sampled 
> values for stime and utime change in flight between task_utime and 
> task_stime are called. Lets say utime will be increased. Given the 
> same sum_exec_runtime that means that the result of task_stime() will 
> get smaller at this point.
> 
> So Chucks patch only deals with sum_exec_runtime changing.

basically sum_exec_runtime is split up to form a precise utime/stime, 
using the stime/utime ratio as the factor.

> > It seems to me that this patch would be the best option for 2.6.23.
> 
> Ingo, do you have any opinion about how to proceed?

the problem occurs when there's a different "split" dictated by 
p->stime/p->utime. The sum of stime+utime as reported should be 
monotonic, but the individual components may not. (the reason is that we 
have a precise "sum" for free, given by the scheduler, but we do not 
want the overhead of per-syscall timestamps to get a precise stime/utime 
numbers. So we sample p->stime and p->utime from the scheduler tick.)

	Ingo
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