linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Sat, 13 Dec 2014 08:43:32 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Frédéric Weisbecker <fweisbec@...il.com>
Cc:	David Lang <david@...g.hm>, Dave Jones <davej@...hat.com>,
	Chris Mason <clm@...com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Dâniel Fraga <fragabr@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: frequent lockups in 3.18rc4


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Fri, Dec 12, 2014 at 11:58 AM, David Lang <david@...g.hm> wrote:
> >
> > If the machine has NOHZ and has a cpu bound userspace task, 
> > it could take quite a while before userspace would trigger a 
> > reschedule (at least if I've understood the comments on this 
> > thread properly)
> 
> The thing is, we'd have to return to user space for that to 
> happen. And when we do that, we check the "should we schedule" 
> flag again. So races like this really shouldn't matter, but 
> there could be something kind-of-similar that just ends up 
> causing a wakeup to be delayed.

Furthermore there ought to be a scheduler tick active in that 
case - which won't be as fast as an immediate reschedule, but 
fast enough to beat the softlockup watchdog's threshold of 20 
seconds or so.

That is why I think it would be interesting to examine how the 
locked up state looks like: is the system truly locked up, 
impossible to log in to, locks held but not released, etc., or is 
the lockup transient?

> But it would need to be delayed for seconds (for the RCU 
> threads) or for tens of seconds (for the watchdog) to matter.
> 
> Which just seems unlikely. Even the "very high load" thing 
> shouldn't really matter, since while that could delay one 
> particular thread being scheduled, it shouldn't delay the next 
> "should we schedule" test. In fact, high load would normally be 
> extected to make the next "should we schedule" come faster.
> 
> But this is where some load calculation overflow might screw 
> things up, of course.

Also, the percpu watchdog threads are SCHED_FIFO:99, woken up 
through percpu hrtimers, which are not easy to delay through high 
SCHED_OTHER load.

Thanks,

	Ingo
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