Messages in this thread Patch in this message | | | From | Valentin Schneider <> | Subject | Re: [PATCH 5/8] sched/fair: Make check_misfit_status() only compare dynamic capacities | Date | Thu, 04 Feb 2021 11:34:38 +0000 |
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On 04/02/21 11:49, Dietmar Eggemann wrote: > On 03/02/2021 16:15, Qais Yousef wrote: >> On 01/28/21 18:31, Valentin Schneider wrote: > > [...] > >>> @@ -10238,7 +10236,7 @@ static void nohz_balancer_kick(struct rq *rq) >>> * When ASYM_CPUCAPACITY; see if there's a higher capacity CPU >>> * to run the misfit task on. >>> */ >>> - if (check_misfit_status(rq, sd)) { >>> + if (check_misfit_status(rq)) { > > Since check_misfit_status() doesn't need sd anymore it looks like that > rcu_dereference(per_cpu(sd_asym_cpucapacity, cpu)) could be replaced by > static_branch_unlikely(&sched_asym_cpucapacity)) in nohz_balancer_kick(). > > But as you mentioned in an earlier conversation we do need to check sd > because of asymmetric CPU capacity systems w/ exclusive cpusets which > could create symmetric islands (unique capacity_orig among CPUs). > > Maybe worth putting a comment here (similar to the one in sis()) so > people don't try to optimize?
How about:
--->8--- diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index c2351b87824f..4b71f4d1d324 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6322,15 +6322,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target) * sd_asym_cpucapacity rather than sd_llc. */ if (static_branch_unlikely(&sched_asym_cpucapacity)) { + /* See sd_has_asym_cpucapacity() */ sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, target)); - /* - * On an asymmetric CPU capacity system where an exclusive - * cpuset defines a symmetric island (i.e. one unique - * capacity_orig value through the cpuset), the key will be set - * but the CPUs within that cpuset will not have a domain with - * SD_ASYM_CPUCAPACITY. These should follow the usual symmetric - * capacity path. - */ if (sd) { i = select_idle_capacity(p, sd, target); return ((unsigned)i < nr_cpumask_bits) ? i : target; @@ -10274,6 +10267,10 @@ static void nohz_balancer_kick(struct rq *rq) } } + /* + * Below checks don't actually use the sd, but they still hinge on its + * presence. See sd_has_asym_cpucapacity(). + */ sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, cpu)); if (sd) { /* diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 21bd71f58c06..ea7f0155e268 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1482,6 +1482,33 @@ DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing); DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_cpucapacity); extern struct static_key_false sched_asym_cpucapacity; +/* + * Note that the static key is system-wide, but the visibility of + * SD_ASYM_CPUCAPACITY isn't. Thus the static key being enabled does not + * imply all CPUs can see asymmetry. + * + * Consider an asymmetric CPU capacity system such as: + * + * MC [ ] + * 0 1 2 3 4 5 + * L L L L B B + * + * w/ arch_scale_cpu_capacity(L) < arch_scale_cpu_capacity(B) + * + * By default, booting this system will enable the sched_asym_cpucapacity + * static key, and all CPUs will see SD_ASYM_CPUCAPACITY set at their MC + * sched_domain. + * + * Further consider exclusive cpusets creating a "symmetric island": + * + * MC [ ][ ] + * 0 1 2 3 4 5 + * L L L L B B + * + * Again, booting this will enable the static key, but CPUs 0-1 will *not* have + * SD_ASYM_CPUCAPACITY set in any of their sched_domain. This is the intending + * behaviour, as CPUs 0-1 should be treated as a regular, isolated SMP system. + */ static inline bool sd_has_asym_cpucapacity(struct sched_domain *sd) { return static_branch_unlikely(&sched_asym_cpucapacity) &&
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