linux-kernel - Re: Bootup time regression from 2.6.27 to 2.6.28-rc3+

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Message-Id: <200811140245.40505.elendil@planet.nl>
Date:	Fri, 14 Nov 2008 02:45:38 +0100
From:	Frans Pop <elendil@...net.nl>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Lukas Hejtmanek <xhejtman@....muni.cz>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Marcin Slusarz <marcin.slusarz@...il.com>,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	corsac@...ian.org
Subject: Re: Bootup time regression from 2.6.27 to 2.6.28-rc3+

Hi Thomas/Arjan,

On Sunday 09 November 2008, Thomas Gleixner wrote:
> I worked all week after returning from travelling to get down to the
> root cause of this. We had several issues:
>
> 1) peek_ahead confused the hell out of the nohz code when it expired
> timers when the timer interrupt was already pending
>
> 2) the changes I did to prevent the stale jiffies in the nohz case
> turned out to be incomplete and wrong versus the broadcast of timers
> in deeper C-States
>
> 3) the consolidation of the nohz idle checks called the nohz jiffies
> update code before calling __irq_enter() which caused useless wakeups
> of ksoftirqd
>
> 4) A long standing - but until now papered over and therefor unnoticed
> - problem of spurious wakeups from deep c-states:
>
>   If one core gets an interrupt the other core(s) wake up as
>   well. The stale jiffies fix and the peek_ahead changes made this
>   more visible and is a major part of the reported nohz (more wakeups,
>   more power consumption) regression.
>
>   This was there forever, but nobody noticed as it was not accounted
>   for.
>
> Find below the lineup of the timers-fixes-for-linus branch of the tip
> tree (the same as Arjan posted minus the irq fixes)

Could either of you maybe give a status update on this patch set and the 
remaining issues with it that were reported (especially the high C0 
reported by powertop)?

TIA,
FJP
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