Re: sched_rt_period_timer causing large latencies

From: Nicholas Piggin
Date: Thu Apr 05 2018 - 06:09:18 EST

On Thu, 05 Apr 2018 10:40:20 +0200
Mike Galbraith <efault@xxxxxx> 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.