Messages in this thread | | | Subject | Re: [PATCH] sched, fair: Allow a small degree of load imbalance between SD_NUMA domains | From | Valentin Schneider <> | Date | Thu, 19 Dec 2019 11:56:09 +0000 |
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On 18/12/2019 22:50, Mel Gorman wrote: >> I'm quite sure you have reasons to have written it that way, but I was >> hoping we could squash it down to something like: > > I wrote it that way to make it clear exactly what has changed, the > thinking behind the checks and to avoid 80-col limits to make review > easier overall. It's a force of habit and I'm happy to reformat it as > you suggest except.... >
I tend to disregard the 80 col limit, so I might not be the best example here :D
>> --- >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index 08a233e97a01..f05d09a8452e 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -8680,16 +8680,27 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s >> env->migration_type = migrate_task; >> lsub_positive(&nr_diff, local->sum_nr_running); >> env->imbalance = nr_diff >> 1; >> - return; >> + } else { >> + >> + /* >> + * If there is no overload, we just want to even the number of >> + * idle cpus. >> + */ >> + env->migration_type = migrate_task; >> + env->imbalance = max_t(long, 0, (local->idle_cpus - >> + busiest->idle_cpus) >> 1); >> } >> >> /* >> - * If there is no overload, we just want to even the number of >> - * idle cpus. >> + * Allow for a small imbalance between NUMA groups; don't do any >> + * of it if there is at least half as many tasks / busy CPUs as >> + * there are available CPUs in the busiest group >> */ >> - env->migration_type = migrate_task; >> - env->imbalance = max_t(long, 0, (local->idle_cpus - >> - busiest->idle_cpus) >> 1); >> + if (env->sd->flags & SD_NUMA && >> + (busiest->sum_nr_running < busiest->group_weight >> 1) && > > This last line is not exactly equivalent to what I wrote. It would need > to be > > (busiest->sum_nr_running < (busiest->group_weight >> 1) - imbalance_adj) && >
Right, I was implicitly suggesting that maybe we could forgo the imbalance_adj computation and just roll with the imbalance_pct (with perhaps and extra shift here and there). IMO the important thing here is the half-way cutoff.
> I can test as you suggest to see if it's roughly equivalent in terms of > performance. The intent was to have a cutoff just before we reached 50% > running tasks / busy CPUs. >
I think that cutoff makes sense; it's also important that it isn't purely busy CPU-based because we're not guaranteed to have 1 task per CPU (due to affinity or else), so I think the "half as many tasks as available CPUs" thing has some merit.
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