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

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Date:	Sat, 3 Mar 2012 00:39:02 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Dan Williams <dan.j.williams@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	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 Wed, Feb 29, 2012 at 10:17:03AM +0100, Peter Zijlstra wrote:
> On Tue, 2012-02-28 at 14:16 -0800, Dan Williams wrote:
> > Looks like everyone is guilty:
> > 
> > [  422.765336] softirq took longer than 1/4 tick: 3 NET_RX ffffffff813f0aa0
> > ...
> > [  423.971878] softirq took longer than 1/4 tick: 4 BLOCK ffffffff812519c8
> > [  423.985093] softirq took longer than 1/4 tick: 6 TASKLET ffffffff8103422e
> > [  423.993157] softirq took longer than 1/4 tick: 7 SCHED ffffffff8105e2e1
> > [  424.001018] softirq took longer than 1/4 tick: 9 RCU ffffffff810a0fed
> > [  424.008691] softirq loop took longer than 1/2 tick need_resched:
> 
> /me kicks himself for not printing the actual duration.. :-)
> 
> > As expected whenever that 1/2 tick message gets emitted the softirq
> > handler is almost running in a need_resched() context.
> 
> Yeah.. that's quite expected.
> 
> > So is it a good idea to get more aggressive about scheduling ksoftrrqd? 
> 
> Nah, moving away from softirq more like. I'll put moving the
> load-balancer into a kthread on the todo list. And it looks like
> everybody else should move to kthreads too.

Last year when I tried that, things got a bit ugly.  I guess I don't need
the kthreads to be realtime unless CONFIG_RCU_BOOST=y, maybe that will
help.  Also IIRC Steven Rostedt made some real-time changes that might
help as well for my case, which would have a per-CPU RT kthread.

							Thanx, Paul

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