On Tue, Jul 14, 2015 at 04:17:46PM +0200, Mike Galbraith wrote: > There's a buglet,
> We'll not look for a idle cpu when wake_wide() naks want_affine. *sigh* indeed.. fixing that'll bring us very close to what we started out wiht.. The one XXX there raises the question on whether we should always so select_idle_sibling() if we do not have a suitable balance flag, or only on wakeups. --- Subject: sched: Beef up wake_wide() From: Mike Galbraith <umgwanakikb...@gmail.com> Date: Fri, 10 Jul 2015 07:19:26 +0200 Josef Bacik reported that Facebook sees better performance with their 1:N load (1 dispatch/node, N workers/node) when carrying an old patch to try very hard to wake to an idle CPU. While looking at wake_wide(), I noticed that it doesn't pay attention to the wakeup of a many partner waker, returning 1 only when waking one of its many partners. Correct that, letting explicit domain flags override the heuristic. While at it, adjust task_struct bits, we don't need a 64bit counter. Cc: morten.rasmus...@arm.com Cc: r...@redhat.com Cc: mi...@redhat.com Signed-off-by: Mike Galbraith <umgwanakikb...@gmail.com> Tested-by: Josef Bacik <jba...@fb.com> [peterz: frobbings] Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> Link: http://lkml.kernel.org/r/1436505566.5715.50.ca...@gmail.com --- include/linux/sched.h | 4 +- kernel/sched/fair.c | 69 ++++++++++++++++++++++++-------------------------- 2 files changed, 36 insertions(+), 37 deletions(-) --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1359,9 +1359,9 @@ struct task_struct { #ifdef CONFIG_SMP struct llist_node wake_entry; int on_cpu; - struct task_struct *last_wakee; - unsigned long wakee_flips; + unsigned int wakee_flips; unsigned long wakee_flip_decay_ts; + struct task_struct *last_wakee; int wake_cpu; #endif --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4726,26 +4726,29 @@ static long effective_load(struct task_g #endif +/* + * Detect M:N waker/wakee relationships via a switching-frequency heuristic. + * A waker of many should wake a different task than the one last awakened + * at a frequency roughly N times higher than one of its wakees. In order + * to determine whether we should let the load spread vs consolodating to + * shared cache, we look for a minimum 'flip' frequency of llc_size in one + * partner, and a factor of lls_size higher frequency in the other. With + * both conditions met, we can be relatively sure that the relationship is + * non-monogamous, with partner count exceeding socket size. Waker/wakee + * being client/server, worker/dispatcher, interrupt source or whatever is + * irrelevant, spread criteria is apparent partner count exceeds socket size. + */ static int wake_wide(struct task_struct *p) { + unsigned int master = current->wakee_flips; + unsigned int slave = p->wakee_flips; int factor = this_cpu_read(sd_llc_size); - /* - * Yeah, it's the switching-frequency, could means many wakee or - * rapidly switch, use factor here will just help to automatically - * adjust the loose-degree, so bigger node will lead to more pull. - */ - if (p->wakee_flips > factor) { - /* - * wakee is somewhat hot, it needs certain amount of cpu - * resource, so if waker is far more hot, prefer to leave - * it alone. - */ - if (current->wakee_flips > (factor * p->wakee_flips)) - return 1; - } - - return 0; + if (master < slave) + swap(master, slave); + if (slave < factor || master < slave * factor) + return 0; + return 1; } static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync) @@ -4757,13 +4760,6 @@ static int wake_affine(struct sched_doma unsigned long weight; int balanced; - /* - * If we wake multiple tasks be careful to not bounce - * ourselves around too much. - */ - if (wake_wide(p)) - return 0; - idx = sd->wake_idx; this_cpu = smp_processor_id(); prev_cpu = task_cpu(p); @@ -5015,19 +5011,19 @@ static int get_cpu_usage(int cpu) static int select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_flags) { - struct sched_domain *tmp, *affine_sd = NULL, *sd = NULL; + struct sched_domain *tmp, affine_sd = NULL, *sd = NULL; int cpu = smp_processor_id(); - int new_cpu = cpu; + int new_cpu = prev_cpu; int want_affine = 0; int sync = wake_flags & WF_SYNC; if (sd_flag & SD_BALANCE_WAKE) - want_affine = cpumask_test_cpu(cpu, tsk_cpus_allowed(p)); + want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, tsk_cpus_allowed(p)); rcu_read_lock(); for_each_domain(cpu, tmp) { if (!(tmp->flags & SD_LOAD_BALANCE)) - continue; + break; /* * If both cpu and prev_cpu are part of this domain, @@ -5041,17 +5037,21 @@ select_task_rq_fair(struct task_struct * if (tmp->flags & sd_flag) sd = tmp; + else if (!want_affine) + break; } - if (affine_sd && cpu != prev_cpu && wake_affine(affine_sd, p, sync)) - prev_cpu = cpu; - - if (sd_flag & SD_BALANCE_WAKE) { - new_cpu = select_idle_sibling(p, prev_cpu); - goto unlock; + if (affine_sd) { + sd = NULL; /* Prefer wake_affine over balance flags */ + if (cpu != prev_cpu && wake_affine(affine_sd, p, sync)) + new_cpu = cpu; } - while (sd) { + if (!sd) { + if (sd_flags & SD_BALANCE_WAKE) /* XXX always ? */ + new_cpu = select_idle_sibling(p, new_cpu); + + } else while (sd) { struct sched_group *group; int weight; @@ -5085,7 +5085,6 @@ select_task_rq_fair(struct task_struct * } /* while loop will break here if sd == NULL */ } -unlock: rcu_read_unlock(); return new_cpu; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
