netdev - Re: 2.6.21-rc1: known regressions (v2) (part 2)

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Date:	Tue, 27 Feb 2007 09:54:13 +0100
From:	Mike Galbraith <efault@....de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Michal Piotrowski <michal.k.k.piotrowski@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Adrian Bunk <bunk@...sta.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"David P. Reed" <dpreed@...d.com>,
	Ayaz Abdulla <aabdulla@...dia.com>, jgarzik@...ox.com,
	netdev@...r.kernel.org,
	Albert Hopkins <kernel@...duk.letterboxes.org>,
	Bob Tracy <rct@...rkin.frus.com>,
	Mark Brown <broonie@...ena.org.uk>,
	"Michael S. Tsirkin" <mst@...lanox.co.il>
Subject: Re: 2.6.21-rc1: known regressions (v2) (part 2)

On Tue, 2007-02-27 at 09:33 +0100, Ingo Molnar wrote:
> * Michal Piotrowski <michal.k.k.piotrowski@...il.com> wrote:
> 
> > Thomas Gleixner napisał(a):
> > > Adrian,
> > > 
> > > On Mon, 2007-02-26 at 23:05 +0100, Adrian Bunk wrote:
> > >> Subject    : kernel BUG at kernel/time/tick-sched.c:168  (CONFIG_NO_HZ)
> > >> References : http://lkml.org/lkml/2007/2/16/346
> > >> Submitter  : Michal Piotrowski <michal.k.k.piotrowski@...il.com>
> > >> Handled-By : Thomas Gleixner <tglx@...utronix.de>
> > >> Status     : problem is being debugged
> > > 
> > > The BUG_ON() was replaced by a warning printk(). The BUG_ON() exposed a
> > > problem with the SMT scheduler. See below.
> > > 
> > >> Subject    : BUG: soft lockup detected on CPU#0
> > >>              NOHZ: local_softirq_pending 20  (SMT scheduler)
> > >> References : http://lkml.org/lkml/2007/2/20/257
> > >> Submitter  : Michal Piotrowski <michal.k.k.piotrowski@...il.com>
> > >> Handled-By : Thomas Gleixner <tglx@...utronix.de>
> > >>              Ingo Molnar <mingo@...e.hu>
> > >> Status     : problem is being debugged
> > > 
> > > Patch available, not confirmed yet.
> > > 
> > 
> > I can confirm that the bug is fixed (over 20 hours of testing should 
> > be enough).
> 
> thanks alot! I think this thing was a long-term performance/latency 
> regression in HT scheduling as well.

Agreed.

I was recently looking at that spot because I found that niced tasks
were taking latency hits, and disabled it, which helped a bunch.  I also
can't understand why it would be OK to interleave a normal task with an
RT task sometimes, but not others.. that's meaningless to the RT task.

IMHO, SMT scheduling should be a buyer beware thing.  Maximizing your
core utilization comes at a price, but so does disabling it, so I think
letting the user decide what he wants is the right thing to do.

	-Mike

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