linux-kernel - Re: sched_rt_period_timer causing large latencies

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Message-ID: <20180405204440.3d5aa162@roar.ozlabs.ibm.com>
Date:   Thu, 5 Apr 2018 20:44:40 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Mike Galbraith <efault@....de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: sched_rt_period_timer causing large latencies

On Thu, 5 Apr 2018 20:08:59 +1000
Nicholas Piggin <npiggin@...il.com> wrote:

> On Thu, 05 Apr 2018 10:40:20 +0200
> Mike Galbraith <efault@....de> wrote:
> 
> > On Thu, 2018-04-05 at 10:27 +0200, Peter Zijlstra wrote:  
> > > On Thu, Apr 05, 2018 at 09:11:38AM +1000, Nicholas Piggin wrote:    
> > > > Hi,
> > > > 
> > > > I'm seeing some pretty big latencies on a ~idle system when a CPU wakes
> > > > out of a nohz idle. Looks like it's due to the taking a lot of remote
> > > > locks and cache lines. irqoff trace:    
> > > 
> > > On RT I think we default RT_RUNTIME_SHARE to false, maybe we should do
> > > the same for mainline.    
> > 
> > Probably.  My very first enterprise encounter with the thing was it NOT
> > saving a box from it's not so clever driver due to that.  
> 
> Well I would think a simpler per-cpu limiter might actually stand a
> better chance of saving you there. Or even something attached to the
> softlockup watchdog.
> 
> I'm still getting a lot of locks coming from sched_rt_period_timer
> with RT_RUNTIME_SHARE false, it's just that it's now down to about
> NR_CPUS locks rather than 3*NR_CPUS.

Oh yeah, putting -1 into sched_rt_runtime_us looks like it fixes it,
I didn't look at Mike's patch close enough.

If the code stays, it would be nice to be able to default it off at
least.

Thanks,
Nick