linux-kernel - Re: [Bug #23902] [BUG] 2.6.37-rc3 massive interactivity regression on ARM

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Message-ID: <19706.39875.954626.75344@pilspetsen.it.uu.se>
Date:	Sat, 4 Dec 2010 20:51:31 +0100
From:	Mikael Pettersson <mikpe@...uu.se>
To:	Mikael Pettersson <mikpe@...uu.se>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Maciej Rutecki <maciej.rutecki@...il.com>,
	Florian Mickler <florian@...kler.org>,
	Venkatesh Pallipadi <venki@...gle.com>
Subject: Re: [Bug #23902] [BUG] 2.6.37-rc3 massive interactivity regression on ARM

Mikael Pettersson writes:
 > Rafael J. Wysocki wrote:
 > > This message has been generated automatically as a part of a summary report
 > > of recent regressions.
 > > 
 > > The following bug entry is on the current list of known regressions
 > > from 2.6.36.  Please verify if it still should be listed and let the tracking team
 > > know (either way).
 > > 
 > > 
 > > Bug-Entry	: http://bugzilla.kernel.org/show_bug.cgi?id=23902
 > > Subject		: [BUG] 2.6.37-rc3 massive interactivity regression on ARM
 > > Submitter	: Mikael Pettersson <mikpe@...uu.se>
 > > Date		: 2010-11-27 15:16 (6 days old)
 > > Message-ID	: <19697.8378.717761.236202@...spetsen.it.uu.se>
 > > References	: http://marc.info/?l=linux-kernel&m=129087098911837&w=2
 > 
 > The bug is still present in 2.6.37-rc4.  I'm currently trying to bisect it.

git bisect identified

[305e6835e05513406fa12820e40e4a8ecb63743c] sched: Do not account irq time to current task

as the cause of this regression.  Reverting it from 2.6.37-rc4 (requires some
hackery due to subsequent changes in the same area) restores sane behaviour.

(Author of original patch cc:d.)

The original patch submission talks about irq-heavy scenarios.  My case is the
exact opposite: UP, !PREEMPT, NO_HZ, very low irq rate, essentially 100% CPU
bound in userspace but expected to schedule quickly when needed (e.g. running
top or ps or just hitting CR in one shell while another runs a compile job).

/Mikael
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