On Thu, 28 Jan 2021 at 19:32, Valentin Schneider <valentin.schnei...@arm.com> wrote: > > Consider a 4-CPU big.LITTLE system with CPUs 0-1 as LITTLEs and CPUs 2-3 as > bigs. The resulting sched_domain hierarchy is: > > DIE [ ] > MC [ ][ ] > 0 1 2 3 > > When running a multithreaded CPU-bound workload (i.e. 1 hog per CPU), the > expected behaviour is to have the about-to-idle big CPUs pull a hog from > the LITTLEs, since bigs will complete their work sooner than LITTLEs. > > Further Consider a scenario where: > - CPU0 is idle (e.g. its hog got migrated to one of the big CPUs) > - CPU1 is currently executing a per-CPU kworker, preempting the CPU hog > - CPU2 and CPU3 are executing CPU-hogs > > CPU0 goes through load_balance() at MC level, and tries to pick stuff from > CPU1, but: > - the hog can't be pulled, because it's task_hot() > - the kworker can't be pulled, because it's pinned to CPU1, which sets > LBF_SOME_PINNED > > This load balance attempts ends with no load pulled, LBF_SOME_PINNED set, > and as a consequence we set the imbalance flag of DIE's [0, 1] > sched_group_capacity. > > Shortly after, CPU2 completes its work and is about to go idle. It goes > through the newidle_balance(), and we would really like it to active > balance the hog running on CPU1 (which is a misfit task). However, > sgc->imbalance is set for the LITTLE group at DIE level, so the group gets > classified as group_imbalanced rather than group_misfit_task. > > Unlike group_misfit_task (via migrate_misfit), the active balance logic > doesn't have any specific case for group_imbalanced, so CPU2 ends up going > idle. We'll have to wait for a load balance on CPU0 or CPU1 to happen and > clear the imbalance flag, and then for another DIE-level load-balance on > CPU2 to happen to pull the task off of CPU1. That's several precious > milliseconds wasted down the drain. > > Giving group_misfit_task a higher group_classify() priority than > group_imbalance doesn't seem like the right thing to do. Instead, make
Could you explain why ? > need_active_balance() return true for any migration_type when the > destination CPU is idle and the source CPU has a misfit task. > > While at it, add an sd_has_asym_cpucapacity() guard in > need_active_balance(). > > Signed-off-by: Valentin Schneider <valentin.schnei...@arm.com> > --- > kernel/sched/fair.c | 13 +++++++++++++ > 1 file changed, 13 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 0ac2f876b86f..cba9f97d9beb 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -9557,9 +9557,22 @@ static int need_active_balance(struct lb_env *env) > return 1; > } > > + if (!sd_has_asym_cpucapacity(sd)) > + return 0; > + > if (env->migration_type == migrate_misfit) > return 1; > > + /* > + * If we failed to pull anything and the src_rq has a misfit task, but > + * the busiest group_type was higher than group_misfit_task, try to > + * go for a misfit active balance anyway. > + */ > + if ((env->idle != CPU_NOT_IDLE) && > + env->src_rq->misfit_task_load && > + cpu_capacity_greater(env->dst_cpu, env->src_cpu)) > + return 1; > + > return 0; > } > > -- > 2.27.0 >