linux-kernel - Re: sched_rt_period_timer causing large latencies

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Date:   Fri, 6 Apr 2018 18:23:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Mike Galbraith <efault@....de>,
        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, Apr 05, 2018 at 08:08:59PM +1000, Nicholas Piggin 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.

Argh, right you are. I had a brief look at what it would take to fix
that, and while it's not completely horrible, it takes a bit of effort.

I'm currently a bit tied up with things, but I'll try not to forget.