Messages in this thread | | | Date | Wed, 12 Feb 2020 14:30:23 +0000 | From | Mel Gorman <> | Subject | Re: [RFC 3/4] sched/fair: replace runnable load average by runnable average |
| |
On Tue, Feb 11, 2020 at 06:46:50PM +0100, Vincent Guittot wrote: > Now that runnable_load_avg is not more used, we can replace it by a new > signal that will highlight the runnable pressure on a cfs_rq. This signal > track the waiting time of tasks on rq and can help to better define the > state of rqs. > > At now, only util_avg is used to define the state of a rq: > A rq with more that around 80% of utilization and more than 1 tasks is > considered as overloaded. > > But the util_avg signal of a rq can become temporaly low after that a task > migrated onto another rq which can bias the classification of the rq. > > When tasks compete for the same rq, their runnable average signal will be > higher than util_avg as it will include the waiting time and we can use > this signal to better classify cfs_rqs. > > The new runnable_avg will track the runnable time of a task which simply > adds the waiting time to the running time. The runnbale _avg of cfs_rq > will be the /Sum of se's runnable_avg and the runnable_avg of group entity > will follow the one of the rq similarly to util_avg. >
s/runnbale/runnable/
Otherwise, all I can do is give a heads-up that I will not be able to review this patch and the next patch properly in the short-term. While the new metric appears to have a sensible definition, I've not spent enough time comparing/contrasting the pro's and con's of PELT implementation details or their consequences. I am not confident I can accurately predict whether this is better or if there are corner cases that make poor placement decisions based on fast changes of runnable_avg. At least not within a reasonable amount of time.
This caught my attention though
> @@ -4065,8 +4018,8 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) > * - Add its new weight to cfs_rq->load.weight > */ > update_load_avg(cfs_rq, se, UPDATE_TG | DO_ATTACH); > + se_update_runnable(se); > update_cfs_group(se); > - enqueue_runnable_load_avg(cfs_rq, se); > account_entity_enqueue(cfs_rq, se); > > if (flags & ENQUEUE_WAKEUP)
I don't think the ordering matters any more because of what was removed from update_cfs_group. Unfortunately, I'm not 100% confident so am bringing it to your attention in case it does.
-- Mel Gorman SUSE Labs
| |