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

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Message-ID: <20170420232746.GF25160@lerouge>
Date:   Fri, 21 Apr 2017 01:27:47 +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 09:40:12PM +0200, Thomas Gleixner wrote:
> On Thu, 20 Apr 2017, Frederic Weisbecker wrote:
> > On Thu, Apr 20, 2017 at 07:56:22PM +0200, Thomas Gleixner wrote:
> > > >  	/* 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?
> 
> 	if (ts->tick_stopped && (expires == ts->next_tick)) {
> 		WARN_ON_ONCE(dev->next_event > ts->next_tick);
> 		goto out;
> 	}
> 
> IOW, the WARN_ON yells in dmesg, but despite seing the wreckage it just
> leaves it and goes out doing nothing.
> 
> Why can't you just do
> 
> 	if (ts->tick_stopped && (expires == ts->next_tick)) {
> 	   	if (dev->next_event > ts->next_tick)) {
> 			WARN_ONCE();
> 			do_something_sensible();
> 		}		
> 		goto out;
> 	}
> 
> Hmm?

Ah ok, right!

So something like this:

    if (ts->tick_stopped && (expires == ts->next_tick)) {
        if (likely(dev->next_event <= ts->next_tick))
            goto out;
        WARN_ON_ONCE(1);
    }

So that we fall down to clock reprogramming if the sanity check fails.

I'm resending the patches.

Thanks.