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

[Frederic Weisbecker]

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.