Messages in this thread | | | Date | Fri, 13 May 2022 12:32:11 +0800 | Subject | Re: [External] Re: [PATCH] sched/fair: optimize and simplify rq leaf_cfs_rq_list | From | Chengming Zhou <> |
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On 2022/5/12 22:02, Vincent Guittot wrote: > Hi Chengming, > > Your patch is on my list but I have been busy on other stuff and we > had enough warnings and problems with this part that I want to > carefully review that all the cases are covered. I will review it soon >
Hi Vincent,
Thanks for your reply. There's no rush, just take your time. I agree that we should be very careful with this part due to previous problems.
Thanks.
> Vincent > > On Wed, 11 May 2022 at 14:05, Chengming Zhou > <zhouchengming@bytedance.com> wrote: >> >> Hello, friendly ping... >> >> >> On 2022/4/28 00:05, Chengming Zhou wrote: >>> We notice the rq leaf_cfs_rq_list has two problems when do bugfix >>> backports and some test profiling. >>> >>> 1. cfs_rqs under throttled subtree could be added to the list, and >>> make their fully decayed ancestors on the list, even though not needed. >>> >>> 2. #1 also make the leaf_cfs_rq_list management complex and error prone, >>> this is the list of related bugfix so far: >>> >>> commit 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()") >>> commit fe61468b2cbc ("sched/fair: Fix enqueue_task_fair warning") >>> commit b34cb07dde7c ("sched/fair: Fix enqueue_task_fair() warning some more") >>> commit 39f23ce07b93 ("sched/fair: Fix unthrottle_cfs_rq() for leaf_cfs_rq list") >>> commit 0258bdfaff5b ("sched/fair: Fix unfairness caused by missing load decay") >>> commit a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle") >>> commit fdaba61ef8a2 ("sched/fair: Ensure that the CFS parent is added after unthrottling") >>> commit 2630cde26711 ("sched/fair: Add ancestors of unthrottled undecayed cfs_rq") >>> >>> commit 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()") >>> delete every cfs_rq under throttled subtree from rq->leaf_cfs_rq_list, >>> and delete the throttled_hierarchy() test in update_blocked_averages(), >>> which optimized update_blocked_averages(). >>> >>> But those later bugfix add cfs_rqs under throttled subtree back to >>> rq->leaf_cfs_rq_list again, with their fully decayed ancestors, for >>> the integrity of rq->leaf_cfs_rq_list. >>> >>> This patch takes another method, skip all cfs_rqs under throttled >>> hierarchy when list_add_leaf_cfs_rq(), to completely make cfs_rqs >>> under throttled subtree off the leaf_cfs_rq_list. >>> >>> So we don't need to consider throttled related things in >>> enqueue_entity(), unthrottle_cfs_rq() and enqueue_task_fair(), >>> which simplify the code a lot. Also optimize update_blocked_averages() >>> since cfs_rqs under throttled hierarchy and their ancestors >>> won't be on the leaf_cfs_rq_list. >>> >>> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com> >>> --- >>> kernel/sched/fair.c | 72 ++++++++++----------------------------------- >>> 1 file changed, 16 insertions(+), 56 deletions(-) >>> >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >>> index 1ad18b5cc1b8..083c3d32c899 100644 >>> --- a/kernel/sched/fair.c >>> +++ b/kernel/sched/fair.c >>> @@ -309,6 +309,8 @@ const struct sched_class fair_sched_class; >>> >>> #ifdef CONFIG_FAIR_GROUP_SCHED >>> >>> +static inline int throttled_hierarchy(struct cfs_rq *cfs_rq); >>> + >>> /* Walk up scheduling entities hierarchy */ >>> #define for_each_sched_entity(se) \ >>> for (; se; se = se->parent) >>> @@ -331,7 +333,7 @@ static inline bool list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq) >>> struct rq *rq = rq_of(cfs_rq); >>> int cpu = cpu_of(rq); >>> >>> - if (cfs_rq->on_list) >>> + if (cfs_rq->on_list || throttled_hierarchy(cfs_rq)) >>> return rq->tmp_alone_branch == &rq->leaf_cfs_rq_list; >>> >>> cfs_rq->on_list = 1; >>> @@ -3242,8 +3244,6 @@ static long calc_group_shares(struct cfs_rq *cfs_rq) >>> } >>> #endif /* CONFIG_SMP */ >>> >>> -static inline int throttled_hierarchy(struct cfs_rq *cfs_rq); >>> - >>> /* >>> * Recomputes the group entity based on the current state of its group >>> * runqueue. >>> @@ -4356,16 +4356,10 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) >>> __enqueue_entity(cfs_rq, se); >>> se->on_rq = 1; >>> >>> - /* >>> - * When bandwidth control is enabled, cfs might have been removed >>> - * because of a parent been throttled but cfs->nr_running > 1. Try to >>> - * add it unconditionally. >>> - */ >>> - if (cfs_rq->nr_running == 1 || cfs_bandwidth_used()) >>> + if (cfs_rq->nr_running == 1) { >>> list_add_leaf_cfs_rq(cfs_rq); >>> - >>> - if (cfs_rq->nr_running == 1) >>> check_enqueue_throttle(cfs_rq); >>> + } >>> } >>> >>> static void __clear_buddies_last(struct sched_entity *se) >>> @@ -4980,11 +4974,18 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq) >>> /* update hierarchical throttle state */ >>> walk_tg_tree_from(cfs_rq->tg, tg_nop, tg_unthrottle_up, (void *)rq); >>> >>> - /* Nothing to run but something to decay (on_list)? Complete the branch */ >>> if (!cfs_rq->load.weight) { >>> - if (cfs_rq->on_list) >>> - goto unthrottle_throttle; >>> - return; >>> + if (!cfs_rq->on_list) >>> + return; >>> + /* >>> + * Nothing to run but something to decay (on_list)? >>> + * Complete the branch. >>> + */ >>> + for_each_sched_entity(se) { >>> + if (list_add_leaf_cfs_rq(cfs_rq_of(se))) >>> + break; >>> + } >>> + goto unthrottle_throttle; >>> } >>> >>> task_delta = cfs_rq->h_nr_running; >>> @@ -5022,31 +5023,12 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq) >>> /* end evaluation on encountering a throttled cfs_rq */ >>> if (cfs_rq_throttled(qcfs_rq)) >>> goto unthrottle_throttle; >>> - >>> - /* >>> - * One parent has been throttled and cfs_rq removed from the >>> - * list. Add it back to not break the leaf list. >>> - */ >>> - if (throttled_hierarchy(qcfs_rq)) >>> - list_add_leaf_cfs_rq(qcfs_rq); >>> } >>> >>> /* At this point se is NULL and we are at root level*/ >>> add_nr_running(rq, task_delta); >>> >>> unthrottle_throttle: >>> - /* >>> - * The cfs_rq_throttled() breaks in the above iteration can result in >>> - * incomplete leaf list maintenance, resulting in triggering the >>> - * assertion below. >>> - */ >>> - for_each_sched_entity(se) { >>> - struct cfs_rq *qcfs_rq = cfs_rq_of(se); >>> - >>> - if (list_add_leaf_cfs_rq(qcfs_rq)) >>> - break; >>> - } >>> - >>> assert_list_leaf_cfs_rq(rq); >>> >>> /* Determine whether we need to wake up potentially idle CPU: */ >>> @@ -5701,13 +5683,6 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags) >>> /* end evaluation on encountering a throttled cfs_rq */ >>> if (cfs_rq_throttled(cfs_rq)) >>> goto enqueue_throttle; >>> - >>> - /* >>> - * One parent has been throttled and cfs_rq removed from the >>> - * list. Add it back to not break the leaf list. >>> - */ >>> - if (throttled_hierarchy(cfs_rq)) >>> - list_add_leaf_cfs_rq(cfs_rq); >>> } >>> >>> /* At this point se is NULL and we are at root level*/ >>> @@ -5731,21 +5706,6 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags) >>> update_overutilized_status(rq); >>> >>> enqueue_throttle: >>> - if (cfs_bandwidth_used()) { >>> - /* >>> - * When bandwidth control is enabled; the cfs_rq_throttled() >>> - * breaks in the above iteration can result in incomplete >>> - * leaf list maintenance, resulting in triggering the assertion >>> - * below. >>> - */ >>> - for_each_sched_entity(se) { >>> - cfs_rq = cfs_rq_of(se); >>> - >>> - if (list_add_leaf_cfs_rq(cfs_rq)) >>> - break; >>> - } >>> - } >>> - >>> assert_list_leaf_cfs_rq(rq); >>> >>> hrtick_update(rq);
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