Re: [PATCH 13/19] mm/migrate: Use xchg instead of spinlock

[Peter Zijlstra]

On Tue, Jun 05, 2018 at 12:24:39AM -0700, Srikar Dronamraju wrote:
> * Peter Zijlstra <pet...@infradead.org> [2018-06-04 21:28:21]:
> 
> > >   if (time_after(jiffies, pgdat->numabalancing_migrate_next_window)) {
> > > -         spin_lock(&pgdat->numabalancing_migrate_lock);
> > > -         pgdat->numabalancing_migrate_nr_pages = 0;
> > > -         pgdat->numabalancing_migrate_next_window = jiffies +
> > > -                 msecs_to_jiffies(migrate_interval_millisecs);
> > > -         spin_unlock(&pgdat->numabalancing_migrate_lock);
> > > +         if (xchg(&pgdat->numabalancing_migrate_nr_pages, 0))
> > > +                 pgdat->numabalancing_migrate_next_window = jiffies +
> > > +                         msecs_to_jiffies(migrate_interval_millisecs);
> >
> > Note that both are in fact wrong. That wants to be something like:
> >
> >     pgdat->numabalancing_migrate_next_window += interval;
> >
> > Otherwise you stretch every interval by 'jiffies - 
> > numabalancing_migrate_next_window'.
> 
> Okay, I get your point.

Note that in practise it probably doesn't matter, but it just upsets my
OCD ;-)

> > Also, that all wants READ_ONCE/WRITE_ONCE, irrespective of the
> > spinlock/xchg.

> unsigned long interval = READ_ONCE(pgdat->numabalancing_migrate_next_window);
> 
> if (time_after(jiffies, interval)) {
>       interval += msecs_to_jiffies(migrate_interval_millisecs));
>       if (xchg(&pgdat->numabalancing_migrate_nr_pages, 0))
>               WRITE_ONCE(pgdat->numabalancing_migrate_next_window, interval);
> }
> 
> Something like this?

Almost, you forgot about the case where 'jiffies -
numabalancing_migrate_next_window > interval'.

That wants to be something like:

  unsigned long timo = READ_ONCE(stupid_long_name);

  if (time_after(jiffies, timo) && xchg(&other_long_name, 0)) {
          do {
                  timo += msec_to_jiffies(..);
          } while (unlikely(time_after(jiffies, timo);

          WRITE_ONCE(stupid_long_name, timo);
  }