Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH 1/8] sched/fair: Clean up active balance nr_balance_failed trickery | Date | Fri, 05 Feb 2021 14:05:35 +0000 |
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On 05/02/21 14:51, Vincent Guittot wrote: > On Thu, 28 Jan 2021 at 19:32, Valentin Schneider > <valentin.schneider@arm.com> 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@arm.com> >> --- >> 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
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().
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