Messages in this thread | | | Subject | Re: [PATCH v2 1/5] sched/fair: Reorder enqueue/dequeue_task_fair path | From | Dietmar Eggemann <> | Date | Thu, 20 Feb 2020 14:38:59 +0100 |
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On 19/02/2020 17:26, Vincent Guittot wrote: > On Wed, 19 Feb 2020 at 12:07, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote: >> >> On 18/02/2020 15:15, Vincent Guittot wrote: >>> On Tue, 18 Feb 2020 at 14:22, Peter Zijlstra <peterz@infradead.org> wrote: >>>> >>>> On Tue, Feb 18, 2020 at 01:37:37PM +0100, Dietmar Eggemann wrote: >>>>> On 14/02/2020 16:27, Vincent Guittot wrote: >>>>>> The walk through the cgroup hierarchy during the enqueue/dequeue of a task >>>>>> is split in 2 distinct parts for throttled cfs_rq without any added value >>>>>> but making code less readable. >>>>>> >>>>>> Change the code ordering such that everything related to a cfs_rq >>>>>> (throttled or not) will be done in the same loop. >>>>>> >>>>>> In addition, the same steps ordering is used when updating a cfs_rq: >>>>>> - update_load_avg >>>>>> - update_cfs_group >>>>>> - update *h_nr_running >>>>> >>>>> Is this code change really necessary? You pay with two extra goto's. We >>>>> still have the two for_each_sched_entity(se)'s because of 'if >>>>> (se->on_rq); break;'. >>>> >>>> IIRC he relies on the presented ordering in patch #5 -- adding the >>>> running_avg metric. >>> >>> Yes, that's the main reason, updating load_avg before h_nr_running >> >> My hunch is you refer to the new function: >> >> static inline void se_update_runnable(struct sched_entity *se) >> { >> if (!entity_is_task(se)) >> se->runnable_weight = se->my_q->h_nr_running; >> } >> >> I don't see the dependency to the 'update_load_avg -> h_nr_running' >> order since it operates on se->my_q, not cfs_rq = cfs_rq_of(se), i.e. >> se->cfs_rq. >> >> What do I miss here? > > update_load_avg() updates both se and cfs_rq so if you update > cfs_rq->h_nr_running before calling update_load_avg() like in the 2nd > for_each_sched_entity, you will update cfs_rq runnable_avg for the > past time slot with the new h_nr_running value instead of the previous > value.
Ah, now I see:
update_load_avg() update_cfs_rq_load_avg() __update_load_avg_cfs_rq() ___update_load_sum(..., cfs_rq->h_nr_running, ...)
^^^^^^^^^^^^^^^^^^^^
Not really obvious IMHO, since the code is introduced only in 4/5.
Could you add a comment to this patch header?
I see you mentioned this dependency already in v1 discussion
https://lore.kernel.org/r/CAKfTPtAM=kgF7Fz-JKFY+s_k5KFirs-8Bub3s1Eqtq7P0NMa0w@mail.gmail.com
"... But the following patches make PELT using h_nr_running ...".
IMHO it would be helpful to have this explanation in the 1/5 patch header so people stop wondering why this is necessary.
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