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

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Date:	Mon, 29 Mar 2010 16:41:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Chase Douglas <chase.douglas@...onical.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: [REGRESSION 2.6.30][PATCH 1/1] sched: defer idle accounting
 till after load update period

On Mon, 2010-03-29 at 09:41 -0400, Chase Douglas wrote:
> There's a period of 10 ticks where calc_load_tasks is updated by all the
> cpus for the load avg. Usually all the cpus do this during the first
> tick. If any cpus go idle, calc_load_tasks is decremented accordingly.
> However, if they wake up calc_load_tasks is not incremented. Thus, if
> cpus go idle during the 10 tick period, calc_load_tasks may be
> decremented to a non-representative value. This issue can lead to
> systems having a load avg of exactly 0, even though the real load avg
> could theoretically be up to NR_CPUS.
> 
> This change defers calc_load_tasks accounting after each cpu updates the
> count until after the 10 tick period.

>>From reading the above changelog it seems to me there should be a
callback from leaving nohz mode, your proposed patch has no such thing.

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