linux-kernel - Re: [REGRESSION 2.6.30][PATCH 1/1] sched: defer idle accounting till after load update period

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Message-ID: <alpine.LFD.2.00.1004012233550.32352@localhost.localdomain>
Date:	Thu, 1 Apr 2010 22:37:39 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Chase Douglas <chase.douglas@...onical.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	kernel-team <kernel-team@...ts.ubuntu.com>
Subject: Re: [REGRESSION 2.6.30][PATCH 1/1] sched: defer idle accounting till
 after load update period

On Thu, 1 Apr 2010, Chase Douglas wrote:

> On Thu, Apr 1, 2010 at 4:18 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > Also we need to think about a more clever way than just accounting the
> > number of running and uninterruptible tasks. What we really want is to
> > use the numbers which the scheduler has handy, i.e how many tasks did
> > we run since we did the last loadavg calculation. It was always
> > possible (even before the big loadavg changes) to create a task which
> > consumes 50% of a CPU and never shows up in the loadavg calculations
> > at all.
> 
> I'm not sure I follow how knowing how many tasks have been run since
> the last LOAD_FREQ expiration will help, or is that just an example of
> the kind of data the scheduler has available that may be useful?

Yes, it's just an example. Look at the following (UP) scenario:

-> tick	calcs loadavg

-> task is woken and runs for 50% of a tick
-> task goes to sleep

-> tick	calcs loadavg

So loadavg will say 0 forever, while we in reality know that there was
significant work on that CPU - at least when we have a sched_clock
which has a better granularity than the tick itself.

Thanks,

	tglx
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