Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 28 Aug 2019 17:02:44 +0200 | Subject | Re: [PATCH 03/15] sched,fair: redefine runnable_load_avg as the sum of task_h_load |
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On Wed, 28 Aug 2019 at 16:48, Rik van Riel <riel@surriel.com> wrote: > > On Wed, 2019-08-28 at 15:50 +0200, Vincent Guittot wrote: > > Hi Rik, > > > > On Thu, 22 Aug 2019 at 04:18, Rik van Riel <riel@surriel.com> wrote: > > > The runnable_load magic is used to quickly propagate information > > > about > > > runnable tasks up the hierarchy of runqueues. The runnable_load_avg > > > is > > > mostly used for the load balancing code, which only examines the > > > value at > > > the root cfs_rq. > > > > > > Redefine the root cfs_rq runnable_load_avg to be the sum of > > > task_h_loads > > > of the runnable tasks. This works because the hierarchical > > > runnable_load of > > > a task is already equal to the task_se_h_load today. This provides > > > enough > > > information to the load balancer. > > > > > > The runnable_load_avg of the cgroup cfs_rqs does not appear to be > > > used for anything, so don't bother calculating those. > > > > > > This removes one of the things that the code currently traverses > > > the > > > cgroup hierarchy for, and getting rid of it brings us one step > > > closer > > > to a flat runqueue for the CPU controller. > > > > I like your proposal but just wanted to clarify one thing with this > > patch. > > Although you removed the computation of runnable_load_avg of the > > cgroup cfs_rq, we are still traversing the hierarchy to update the > > root cfs_rq runnable_load_avg because we are traversing the hierarchy > > for computing the task_h_loads > > The task_h_load hierarchy traversal in update_cfs_rq_h_load > is rate limited to once a jiffy, though. Rate limiting the
Ah yes. I forgot that it was jiffies and not clock_task that is used for limiting the update
> hierarchy traversal significantly reduces overhead. > > > That being said, if we manage to remove the need on using > > runnable_load_avg we will completely skip this traversal. I have a > > proposal to remove it from load balance and wake up path but i > > haven't > > look at numa stats which also use it > > -- > All Rights Reversed.
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