linux-kernel - Re: [RFC PATCH] kick ksoftirqd more often to please soft lockup detector

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Date:	Tue, 28 Feb 2012 22:41:39 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	Dan Williams <dan.j.williams@...el.com>,
	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
	linux-scsi@...r.kernel.org, Lukasz Dorau <lukasz.dorau@...el.com>,
	James Bottomley <JBottomley@...allels.com>,
	Andrzej Jakowski <andrzej.jakowski@...el.com>
Subject: Re: [RFC PATCH] kick ksoftirqd more often to please soft lockup
 detector

On Tue, 28 Feb 2012, Peter Zijlstra wrote:

> On Mon, 2012-02-27 at 12:38 -0800, Dan Williams wrote:
> > An experimental hack to tease out whether we are continuing to
> > run the softirq handler past the point of needing scheduling.
> > 
> > It allows only one trip through __do_softirq() as long as need_resched()
> > is set which hopefully creates the back pressure needed to get ksoftirqd
> > scheduled.
> > 
> > Targeted to address reports like the following that are produced
> > with i/o tests to a sas domain with a large number of disks (48+), and
> > lots of debugging enabled (slub_deubg, lockdep) that makes the
> > block+scsi softirq path more cpu-expensive than normal.
> > 
> > With this patch applied the softlockup detector seems appeased, but it
> > seems odd to need changes to kernel/softirq.c so maybe I have overlooked
> > something that needs changing at the block/scsi level?
> > 
> > BUG: soft lockup - CPU#3 stuck for 22s! [kworker/3:1:78] 
> 
> So you're stuck in softirq for 22s+, max_restart is 10, this gives that
> on average you spend 2.2s+ per softirq invocation, this is completely
> absolutely bonkers. Softirq handlers should never consume significant
> amount of cpu-time.
> 
> Thomas, think its about time we put something like the below in?

Absolutely. Anything which consumes more than a few microseconds in
the softirq handler needs to be sorted out, no matter what.
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