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

From: Pavan Kondeti <pkondeti@codeaurora.org>
To: Qais Yousef <qais.yousef@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] sched/rt: fix pushing unfit tasks to a better CPU
Date: Wed, 19 Feb 2020 08:16:09 +0530	[thread overview]
Message-ID: <20200219024608.GE28029@codeaurora.org> (raw)
In-Reply-To: <20200218174718.ma6cpr2qwnueertn@e107158-lin.cambridge.arm.com>

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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