Re: [PATCH 2/2] sched/fair: Adjust the allowed NUMA imbalance when SD_NUMA spans multiple LLCs

[Mel Gorman <mgorman@techsingularity.net>]

On Wed, Jan 05, 2022 at 10:42:07AM +0000, Mel Gorman wrote:
> On Tue, Dec 21, 2021 at 06:13:15PM +0100, Vincent Guittot wrote:
> > > <SNIP>
> > >
> > > @@ -9050,9 +9054,9 @@ static bool update_pick_idlest(struct sched_group *idlest,
> > >   * This is an approximation as the number of running tasks may not be
> > >   * related to the number of busy CPUs due to sched_setaffinity.
> > >   */
> > > -static inline bool allow_numa_imbalance(int dst_running, int dst_weight)
> > > +static inline bool allow_numa_imbalance(int dst_running, int imb_numa_nr)
> > >  {
> > > -       return (dst_running < (dst_weight >> 2));
> > > +       return dst_running < imb_numa_nr;
> > >  }
> > >
> > >  /*
> > >
> > > <SNIP>
> > >
> > > @@ -9280,19 +9285,13 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> > >         }
> > >  }
> > >
> > > -#define NUMA_IMBALANCE_MIN 2
> > > -
> > >  static inline long adjust_numa_imbalance(int imbalance,
> > > -                               int dst_running, int dst_weight)
> > > +                               int dst_running, int imb_numa_nr)
> > >  {
> > > -       if (!allow_numa_imbalance(dst_running, dst_weight))
> > > +       if (!allow_numa_imbalance(dst_running, imb_numa_nr))
> > >                 return imbalance;
> > >
> > > -       /*
> > > -        * Allow a small imbalance based on a simple pair of communicating
> > > -        * tasks that remain local when the destination is lightly loaded.
> > > -        */
> > > -       if (imbalance <= NUMA_IMBALANCE_MIN)
> > > +       if (imbalance <= imb_numa_nr)
> > 
> > Isn't this always true ?
> > 
> > imbalance is "always" < dst_running as imbalance is usually the number
> > of these tasks that we would like to migrate
> > 
> 
> It's not necessarily true. allow_numa_imbalanced is checking if
> dst_running < imb_numa_nr and adjust_numa_imbalance is checking the
> imbalance.
> 
> imb_numa_nr = 4
> dst_running = 2
> imbalance   = 1
> 
> In that case, imbalance of 1 is ok, but 2 is not.
> 

My bad, this is based on v5 which I just queued for testing.

-- 
Mel Gorman
SUSE Labs