linux-kernel - Re: [ANNOUNCE] 4.4-rc6-rt1

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Date:	Thu, 14 Jan 2016 16:07:39 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
cc:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Clark Williams <williams@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [ANNOUNCE] 4.4-rc6-rt1

On Thu, 14 Jan 2016, Mike Galbraith wrote:

> On Thu, 2016-01-14 at 15:30 +0100, Thomas Gleixner wrote:
> > On Thu, 14 Jan 2016, Mike Galbraith wrote:
> > 
> > > On Thu, 2016-01-14 at 15:17 +0100, Sebastian Andrzej Siewior wrote:
> > > > * Sebastian Andrzej Siewior | 2016-01-13 18:58:45 [+0100]:
> > > > 
> > > > > This is due to NO_HZ as far as I can tell. My AMD A10 in idle
> > mode
> > > > > has
> > > > > 0.7% utilisation of ksoftirqd/ with CONFIG_HZ_PERIODIC and with
> > > > > CONFIG_NO_HZ_FULL it shows about 25% on all CPU threads.
> > > > 
> > > > This should fixed it:
> > > > 
> > > > --- a/kernel/time/timer.c
> > > > +++ b/kernel/time/timer.c
> > > > @@ -1453,7 +1453,7 @@ u64 get_next_timer_interrupt(unsigned long
> > > > basej, u64 basem)
> > > >      * the base lock to check when the next timer is pending and
> > > > so
> > > >      * we assume the next jiffy.
> > > >      */
> > > > -   return basej;
> > > > +   return basem + TICK_NSEC;
> > > >  #endif
> > > >     spin_lock(&base->lock);
> > > >     if (base->active_timers) {
> > > 
> > > That's what I had done to stop the screaming interrupt, but box
> > still
> > > behaved very badly.
> > 
> > If you turn off CONFIG_NO_HZ_FULL and switch to NO_HZ_IDLE is it
> > still bad?
> 
> I didn't have CONFIG_NO_HZ_FULL enabled, it was CONFIG_NO_HZ_IDLE.

So with the above fix it still behaves badly. Can you provide your config and
a hint which workload/idle/whatever state results in bad behaviour.

Thanks,

	tglx