Messages in this thread Patch in this message | | | Date | Wed, 19 Feb 2020 08:16:09 +0530 | From | Pavan Kondeti <> | Subject | Re: [PATCH 3/3] sched/rt: fix pushing unfit tasks to a better CPU |
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On Tue, Feb 18, 2020 at 05:47:19PM +0000, Qais Yousef wrote: > On 02/18/20 09:46, Pavan Kondeti wrote: > > The original RT task placement i.e without capacity awareness, places the task > > on the previous CPU if the task can preempt the running task. I interpreted it > > as that "higher prio RT" task should get better treatment even if it results > > in stopping the lower prio RT execution and migrating it to another CPU. > > > > Now coming to your patch (merged), we force find_lowest_rq() if the previous > > CPU can't fit the task though this task can right away run there. When the > > lowest mask returns an unfit CPU (with your new patch), We have two choices, > > either to place it on this unfit CPU (may involve migration) or place it on > > the previous CPU to avoid the migration. We are selecting the first approach. > > > > The task_cpu(p) check in find_lowest_rq() only works when the previous CPU > > does not have a RT task. If it is running a lower prio RT task than the > > waking task, the lowest_mask may not contain the previous CPU. > > > > I don't if any workload hurts due to this change in behavior. So not sure > > if we have to restore the original behavior. Something like below will do. > > Is this patch equivalent to yours? If yes, then I got you. If not, then I need > to re-read this again.. > > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c > index ace9acf9d63c..854a0c9a7be6 100644 > --- a/kernel/sched/rt.c > +++ b/kernel/sched/rt.c > @@ -1476,6 +1476,13 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags) > if (test || !rt_task_fits_capacity(p, cpu)) { > int target = find_lowest_rq(p); > > + /* > + * Bail out if we were forcing a migration to find a better > + * fitting CPU but our search failed. > + */ > + if (!test && !rt_task_fits_capacity(p, target)) > + goto out_unlock; > +
Yes. This is what I was referring to.
Thanks, Pavan
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