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Date:   Mon, 23 Sep 2019 12:52:24 +0100
From:   Qais Yousef <qais.yousef@arm.com>
To:     Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Alessio Balsini <balsini@android.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched: rt: Make RT capacity aware
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On 09/20/19 14:52, Dietmar Eggemann wrote:
> > 	2. The fallback mechanism means we either have to call cpupri_find()
> > 	   twice once to find filtered lowest_rq and the other to return the
> > 	   none filtered version.
> 
> This is what I have in mind. (Only compile tested! ... and the 'if
> (cpumask_any(lowest_mask) >= nr_cpu_ids)' condition has to be considered
> as well):
> 
> @@ -98,8 +103,26 @@ int cpupri_find(struct cpupri *cp, struct
> task_struct *p,
>                         continue;
> 
>                 if (lowest_mask) {
> +                       int cpu, max_cap_cpu = -1;
> +                       unsigned long max_cap = 0;
> +
>                         cpumask_and(lowest_mask, p->cpus_ptr, vec->mask);
> 
> +                       for_each_cpu(cpu, lowest_mask) {
> +                               unsigned long cap =
> arch_scale_cpu_capacity(cpu);
> +
> +                               if (!rt_task_fits_capacity(p, cpu))
> +                                       cpumask_clear_cpu(cpu, lowest_mask);
> +
> +                               if (cap > max_cap) {
> +                                       max_cap = cap;
> +                                       max_cap_cpu = cpu;
> +                               }
> +                       }
> +
> +                       if (cpumask_empty(lowest_mask) && max_cap)
> +                               cpumask_set_cpu(max_cap_cpu, lowest_mask);

I had a patch that I was testing but what I did is to continue rather than
return a max_cap_cpu.

e.g:

	if no cpu at current priority fits the task:
		continue;
	else:
		return the lowest_mask which contains fitting cpus only

	if no fitting cpu was find:
		return 0;


Or we can tweak your approach to be

	if no cpu at current priority fits the task:
		if the cpu the task is currently running on doesn't fit it:
			return lowest_mask with max_cap_cpu set;

So we either:

	1. Continue the search until we find a fitting CPU; bail out otherwise.

	2. Or we attempt to return a CPU only if the CPU the task is currently
	   running on doesn't fit it. We don't want to migrate the task from a
	   fitting to a non-fitting one.

We can also do something hybrid like:

	3. Remember the outcome of 2 but don't return immediately and attempt
	   to find a fitting CPU at a different priority level.


Personally I see 1 is the simplest and good enough solution. What do you think?

I think this is 'continue' to search makes doing it at cpupri_find() more
robust than having to deal with whatever mask we first found in
find_lowest_rq() - so I'm starting to like this approach better. Thanks for
bringing it up.


Cheers

--
Qais Yousef