linux-kernel - Re: [PATCH 2/2] tick: Make sure tick timer is active when bypassing reprogramming

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Date:   Thu, 20 Apr 2017 20:29:02 +0200
From:   Frederic Weisbecker <fweisbec@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...hat.com>,
        James Hartsock <hartsjc@...hat.com>, stable@...r.kernel.org,
        Tim Wright <tim@...bash.co.uk>, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH 2/2] tick: Make sure tick timer is active when bypassing
 reprogramming

On Thu, Apr 20, 2017 at 07:56:22PM +0200, Thomas Gleixner wrote:
> On Thu, 20 Apr 2017, Frederic Weisbecker wrote:
> 
> > So far we have run into too much troubles with the optimization path
> > that skips reprogramming the clock on IRQ exit when the expiration
> > deadline hasn't changed. If by accident the cached deadline happens to
> > be out of sync with the hardware deadline, the buggy result and its
> > cause are hard to investigate. So lets detect and warn about the issue
> > early.
> > 
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > Cc: Tim Wright <tim@...bash.co.uk>
> > Cc: Pavel Machek <pavel@....cz>
> > Cc: James Hartsock <hartsjc@...hat.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Rik van Riel <riel@...hat.com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > ---
> >  kernel/time/tick-sched.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 502b320..eb1366e 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -783,8 +783,10 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
> >  	tick = expires;
> >  
> >  	/* Skip reprogram of event if its not changed */
> > -	if (ts->tick_stopped && (expires == ts->next_tick))
> > +	if (ts->tick_stopped && (expires == ts->next_tick)) {
> > +		WARN_ON_ONCE(dev->next_event > ts->next_tick);
> 
> What about handling it proper ? dev->next_event might be KTIME_MAX,
> i.e. no more event for the next 500+ years.

I thought I handled this case, what I'm I missing?

> Thanks,
> 
> 	tglx