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SubjectRe: [PATCH 3/3] sched/rt: fix pushing unfit tasks to a better CPU
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;
+
/*
* Don't bother moving it if the destination CPU is
* not running a lower priority task.
@@ -1484,6 +1491,8 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
p->prio < cpu_rq(target)->rt.highest_prio.curr)
cpu = target;
}
+
+out_unlock:
rcu_read_unlock();

out:

>
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 4043abe..c80d948 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1475,11 +1475,15 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
> int target = find_lowest_rq(p);
>
> /*
> - * Don't bother moving it if the destination CPU is
> - * not running a lower priority task.
> + * Don't bother moving it
> + *
> + * - If the destination CPU is not running a lower priority task
> + * - The task can't fit on the destination CPU and it can run
> + * right away on it's previous CPU.
> */
> - if (target != -1 &&
> - p->prio < cpu_rq(target)->rt.highest_prio.curr)
> + if (target != -1 && target != cpu &&
> + p->prio < cpu_rq(target)->rt.highest_prio.curr &&
> + (test || rt_task_fits_capacity(p, target)))
> cpu = target;
> }
> rcu_read_unlock();
>
> Thanks,
> Pavan
>
> --
> Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.

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