Re: [PATCH 3/3] sched/rt: fix pushing unfit tasks to a better CPU

[Qais Yousef <qais.yousef@arm.com>]

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.
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