From: Valentin Schneider <valentin.schneider@arm.com>
To: Quentin Perret <qperret@google.com>
Cc: linux-kernel@vger.kernel.org, peterz@infradead.org,
mingo@kernel.org, vincent.guittot@linaro.org,
dietmar.eggemann@arm.com, patrick.bellasi@matbug.net,
qais.yousef@arm.com, morten.rasmussen@arm.com
Subject: Re: [PATCH 3/3] sched/fair: Consider uclamp for "task fits capacity" checks
Date: Thu, 21 Nov 2019 14:51:06 +0000 [thread overview]
Message-ID: <09e234a2-ea65-4d09-5215-9b0fe4ec09fe@arm.com> (raw)
In-Reply-To: <20191121133043.GA46904@google.com>
On 21/11/2019 13:30, Quentin Perret wrote:
> On Thursday 21 Nov 2019 at 12:56:39 (+0000), Valentin Schneider wrote:
>>> @@ -6274,6 +6274,15 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>>> if (!fits_capacity(util, cpu_cap))
>>> continue;
>>>
>>> + /*
>>> + * Skip CPUs that don't satisfy uclamp requests. Note
>>> + * that the above already ensures the CPU has enough
>>> + * spare capacity for the task; this is only really for
>>> + * uclamp restrictions.
>>> + */
>>> + if (!task_fits_capacity(p, capacity_orig_of(cpu)))
>>> + continue;
>>
>> This is partly redundant with the above, I think. What we really want here
>> is just
>>
>> fits_capacity(uclamp_eff_value(p, UCLAMP_MIN), capacity_orig_of(cpu))
>>
>> but this would require some inline #ifdeffery.
>
> This suggested change lacks the UCLAMP_MAX part, which is a shame
> because this is precisely in the EAS path that we should try and
> down-migrate tasks if they have an appropriate max_clamp. So, your first
> proposal made sense, IMO.
>
Hm right, had to let that spin in my head for a while but I think I got it.
I was only really thinking of:
(h960: LITTLE = 462 cap, big = 1024)
p.uclamp.min = 512 -> skip LITTLEs regardless of the actual util_est
but your point is we also want stuff like:
p.uclamp.max = 300 -> accept LITTLEs regardless of the actual util_est
I'll keep the feec() change as-is and add something like the above in the
changelog for v2.
> Another option to avoid the redundancy would be to do something along
> the lines of the totally untested diff below.
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 69a81a5709ff..38cb5fe7ba65 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6372,9 +6372,12 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> if (!cpumask_test_cpu(cpu, p->cpus_ptr))
> continue;
>
> - /* Skip CPUs that will be overutilized. */
> util = cpu_util_next(cpu, p, cpu);
> cpu_cap = capacity_of(cpu);
> + spare_cap = cpu_cap - util;
> + util = uclamp_util_with(cpu_rq(cpu), util, p);
> +
> + /* Skip CPUs that will be overutilized. */
> if (!fits_capacity(util, cpu_cap))
> continue;
>
> @@ -6389,7 +6392,6 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> * Find the CPU with the maximum spare capacity in
> * the performance domain
> */
> - spare_cap = cpu_cap - util;
> if (spare_cap > max_spare_cap) {
> max_spare_cap = spare_cap;
> max_spare_cap_cpu = cpu;
>
> Thoughts ?
>
uclamp_util_with() (or uclamp_rq_util_with() ;)) picks the max between the
rq-aggregated clamps and the task clamps, which isn't what we want. If the
task has a low-ish uclamp.max (e.g. the 300 example from above) but the
rq-wide max-aggregated uclamp.max is ~800, we'd clamp using that 800. It
makes sense for frequency selection, but not for task placement IMO.
> Thanks,
> Quentin
>
next prev parent reply other threads:[~2019-11-21 14:51 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-20 17:55 [PATCH 0/3] sched/fair: Task placement biasing using uclamp Valentin Schneider
2019-11-20 17:55 ` [PATCH 1/3] sched/uclamp: Make uclamp_util_*() helpers use and return UL values Valentin Schneider
2019-11-20 17:55 ` [PATCH 2/3] sched/uclamp: Rename uclamp_util_*() into uclamp_rq_util_*() Valentin Schneider
2019-11-20 17:55 ` [PATCH 3/3] sched/fair: Consider uclamp for "task fits capacity" checks Valentin Schneider
2019-11-21 11:56 ` Quentin Perret
2019-11-21 12:56 ` Valentin Schneider
2019-11-21 13:30 ` Quentin Perret
2019-11-21 14:51 ` Valentin Schneider [this message]
2019-11-21 15:30 ` Quentin Perret
2019-11-21 17:22 ` Valentin Schneider
2019-11-24 22:20 ` Qais Yousef
2019-11-25 17:33 ` Valentin Schneider
2019-11-26 10:06 ` Qais Yousef
2019-11-21 12:00 ` [PATCH 0/3] sched/fair: Task placement biasing using uclamp Quentin Perret
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