linux-kernel - Re: [PATCH 0/2] nohz: Deal with clock reprogram skipping issues v2

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Date:   Wed, 26 Apr 2017 20:49:13 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Frederic Weisbecker <fweisbec@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...hat.com>,
        James Hartsock <hartsjc@...hat.com>,
        Tim Wright <tim@...bash.co.uk>, Pavel Machek <pavel@....cz>,
        Mike Galbraith <efault@....de>
Subject: Re: [PATCH 0/2] nohz: Deal with clock reprogram skipping issues v2


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> On Mon, Apr 24, 2017 at 04:45:23PM +0200, Ingo Molnar wrote:
> > 
> > * Frederic Weisbecker <fweisbec@...il.com> wrote:
> > 
> > > On Mon, Apr 24, 2017 at 10:08:35AM +0200, Ingo Molnar wrote:
> > > > 
> > > > * Frederic Weisbecker <fweisbec@...il.com> wrote:
> > > > 
> > > > > As suggested by Thomas Gleixner, the second patch now integrates
> > > > > a fix in case the sanity check fails and the clockevent isn't programmed
> > > > > as expected.
> > > > > 
> > > > > Frederic Weisbecker (2):
> > > > >   nohz: Fix again collision between tick and other hrtimers
> > > > >   tick: Make sure tick timer is active when bypassing reprogramming
> > > > > 
> > > > >  kernel/time/tick-sched.c | 33 ++++++++++++++++++++++++++++++---
> > > > >  kernel/time/tick-sched.h |  2 ++
> > > > >  2 files changed, 32 insertions(+), 3 deletions(-)
> > > > 
> > > > So I think one of these is causing a new warning on latest -tip:
> > > > 
> > > > [  333.341756] ------------[ cut here ]------------
> > > > [  333.346404] WARNING: CPU: 0 PID: 0 at kernel/time/tick-sched.c:874 __tick_nohz_idle_enter+0x461/0x490
> > > 
> > > Oh I'll never be done with that bug :)
> > > 
> > > Ok I just booted your config with tip/master and didn't see the warning.
> > > But the boot seem to be stalled some time after mounting the root fs.
> > > 
> > > Can you please try the following patch and tell me what it returns to you?
> > > 
> > > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > > index c47d135..6d72e8b 100644
> > > --- a/kernel/time/tick-sched.c
> > > +++ b/kernel/time/tick-sched.c
> > > @@ -872,6 +872,7 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
> > >  			goto out;
> > >  
> > >  		WARN_ON_ONCE(1);
> > > +		printk_once("basemono: %llu ts->next_tick: %llu dev->next_event: %llu\n", basemono, ts->next_tick, dev->next_event);
> > >  	}
> > >  
> > 
> > Here's what it prints:
> > 
> > [  707.251791] basemono: 706016000000 ts->next_tick: 693216000000 dev->next_event: 706016406127
> 
> So weird...
> 
> Ok I'm going to need serious traces. Can you please add this boot option?
> 
>     trace_event=hrtimer_cancel,hrtimer_start,hrtimer_expire_entry

Sorry, don't have the time for extensive traces this close to the merge window - 
but are you sure you cannot reproduce it?

The warning popped up on all 3 test systems I tried (two Intel servers, one AMD 
server), and it also hit Mike's server - with a fairly regular distro-ish config.

Thanks,

	Ingo