Re: [PATCH 1/8] sched/fair: Clean up active balance nr_balance_failed trickery

[Vincent Guittot]

On Fri, 5 Feb 2021 at 15:05, Valentin Schneider
<valentin.schneider@xxxxxxx> wrote:
>
> On 05/02/21 14:51, Vincent Guittot wrote:
> > On Thu, 28 Jan 2021 at 19:32, Valentin Schneider
> > <valentin.schneider@xxxxxxx> wrote:
> >>
> >> When triggering an active load balance, sd->nr_balance_failed is set to
> >> such a value that any further can_migrate_task() using said sd will ignore
> >> the output of task_hot().
> >>
> >> This behaviour makes sense, as active load balance intentionally preempts a
> >> rq's running task to migrate it right away, but this asynchronous write is
> >> a bit shoddy, as the stopper thread might run active_load_balance_cpu_stop
> >> before the sd->nr_balance_failed write either becomes visible to the
> >> stopper's CPU or even happens on the CPU that appended the stopper work.
> >>
> >> Add a struct lb_env flag to denote active balancing, and use it in
> >> can_migrate_task(). Remove the sd->nr_balance_failed write that served the
> >> same purpose.
> >>
> >> Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>
> >> ---
> >>  kernel/sched/fair.c | 17 ++++++++++-------
> >>  1 file changed, 10 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> index 197a51473e0c..0f6a4e58ce3c 100644
> >> --- a/kernel/sched/fair.c
> >> +++ b/kernel/sched/fair.c
> >> @@ -7423,6 +7423,7 @@ enum migration_type {
> >>  #define LBF_SOME_PINNED        0x08
> >>  #define LBF_NOHZ_STATS 0x10
> >>  #define LBF_NOHZ_AGAIN 0x20
> >> +#define LBF_ACTIVE_LB  0x40
> >>
> >>  struct lb_env {
> >>         struct sched_domain     *sd;
> >> @@ -7608,10 +7609,14 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
> >>
> >>         /*
> >>          * Aggressive migration if:
> >> -        * 1) destination numa is preferred
> >> -        * 2) task is cache cold, or
> >> -        * 3) too many balance attempts have failed.
> >> +        * 1) active balance
> >> +        * 2) destination numa is preferred
> >> +        * 3) task is cache cold, or
> >> +        * 4) too many balance attempts have failed.
> >>          */
> >> +       if (env->flags & LBF_ACTIVE_LB)
> >> +               return 1;
> >> +
> >
> > This changes the behavior for numa system because it skips
> > migrate_degrades_locality() which can return 1 and prevent active
> > migration whatever nr_balance_failed
> >
> > Is that intentional ?
> >
>
> If I read this right, the result of migrate_degrades_locality() is
> (currently) ignored if
>
>   env->sd->nr_balance_failed > env->sd->cache_nice_tries

You're right, I have misread the || condition

>
> While on the load_balance() side, we have:
>
>   /* We've kicked active balancing, force task migration. */
>   sd->nr_balance_failed = sd->cache_nice_tries+1;
>
> So we should currently be ignoring migrate_degrades_locality() in the
> active balance case - what I wrote in the changelog for task_hot() still
> applies to migrate_degrades_locality().