linux-kernel - Re: [PATCH] soft lockup: kill realtime threads before panic

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Message-ID: <20150722051827.GD23662@Sligo.logfs.org>
Date:	Tue, 21 Jul 2015 22:18:27 -0700
From:	Jörn Engel <joern@...estorage.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Spencer Baugh <sbaugh@...ern.com>, Don Zickus <dzickus@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ulrich Obergfell <uobergfe@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Jones <drjones@...hat.com>,
	chai wen <chaiw.fnst@...fujitsu.com>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Stephane Eranian <eranian@...gle.com>,
	open list <linux-kernel@...r.kernel.org>,
	Spencer Baugh <Spencer.baugh@...estorage.com>,
	Joern Engel <joern@...fs.org>
Subject: Re: [PATCH] soft lockup: kill realtime threads before panic

On Wed, Jul 22, 2015 at 06:36:30AM +0200, Mike Galbraith wrote:
> On Tue, 2015-07-21 at 15:07 -0700, Spencer Baugh wrote:
> 
> > We have observed cases where the soft lockup detector triggered, but no
> > kernel bug existed.  Instead we had a buggy realtime thread that
> > monopolized a cpu.  So let's kill the responsible party and not panic
> > the entire system.
> 
> If you don't tell the kernel to panic, it won't, and if you don't remove
> its leash (the throttle), your not so tame rt beast won't maul you.

Not sure if this patch is something for mainline, but those two
alternatives have problems of their own.  Not panicking on lockups can
leave a system disabled until some human come around.  In many cases
that human will do no better than power-cycle.  A panic reduces the
downtime.

And the realtime throttling gives non-realtime threads some minimum
runtime, but does nothing to help low-priority realtime threads.  It
also introduces latencies, often when workloads are high and you would
like any available cpu to get through that rough spot.

I don't think we have a good answer to this problem in the mainline
kernel yet.

Jörn

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