Re: [PATCH] Revert "cpufreq: schedutil: Don't restrict kthread to related_cpus unnecessarily"

From: Viresh Kumar
Date: Wed May 09 2018 - 00:55:35 EST


On 08-05-18, 08:33, Dietmar Eggemann wrote:
> This reverts commit e2cabe48c20efb174ce0c01190f8b9c5f3ea1d13.
>
> Lifting the restriction that the sugov kthread is bound to the
> policy->related_cpus for a system with a slow switching cpufreq driver,
> which is able to perform DVFS from any cpu (e.g. cpufreq-dt), is not
> only not beneficial it also harms Enery-Aware Scheduling (EAS) on
> systems with asymmetric cpu capacities (e.g. Arm big.LITTLE).
>
> The sugov kthread which does the update for the little cpus could
> potentially run on a big cpu. It could prevent that the big cluster goes
> into deeper idle states although all the tasks are running on the little
> cluster.
>
> Example: hikey960 w/ 4.16.0-rc6-+
> Arm big.LITTLE with per-cluster DVFS
>
> root@h960:~# cat /proc/cpuinfo | grep "^CPU part"
> CPU part : 0xd03 (Cortex-A53, little cpu)
> CPU part : 0xd03
> CPU part : 0xd03
> CPU part : 0xd03
> CPU part : 0xd09 (Cortex-A73, big cpu)
> CPU part : 0xd09
> CPU part : 0xd09
> CPU part : 0xd09
>
> root@h960:/sys/devices/system/cpu/cpufreq# ls
> policy0 policy4 schedutil
>
> root@h960:/sys/devices/system/cpu/cpufreq# cat policy*/related_cpus
> 0 1 2 3
> 4 5 6 7
>
> (1) w/o the revert:
>
> root@h960:~# ps -eo pid,class,rtprio,pri,psr,comm | awk 'NR == 1 ||
> /sugov/'
> PID CLS RTPRIO PRI PSR COMMAND
> 1489 #6 0 140 1 sugov:0
> 1490 #6 0 140 0 sugov:4
>
> The sugov kthread sugov:4 responsible for policy4 runs on cpu0. (In this
> case both sugov kthreads run on little cpus).
>
> cross policy (cluster) remote callback example:
> ...
> migration/1-14 [001] enqueue_task_fair: this_cpu=1 cpu_of(rq)=5
> migration/1-14 [001] sugov_update_shared: this_cpu=1 sg_cpu->cpu=5
> sg_cpu->sg_policy->policy->related_cpus=4-7
> sugov:4-1490 [000] sugov_work: this_cpu=0
> sg_cpu->sg_policy->policy->related_cpus=4-7
> ...
>
> The remote callback (this_cpu=1, target_cpu=5) is executed on cpu=0.
>
> (2) w/ the revert:
>
> root@h960:~# ps -eo pid,class,rtprio,pri,psr,comm | awk 'NR == 1 ||
> /sugov/'
> PID CLS RTPRIO PRI PSR COMMAND
> 1491 #6 0 140 2 sugov:0
> 1492 #6 0 140 4 sugov:4
>
> The sugov kthread sugov:4 responsible for policy4 runs on cpu4.
>
> cross policy (cluster) remote callback example:
> ...
> migration/1-14 [001] enqueue_task_fair: this_cpu=1 cpu_of(rq)=7
> migration/1-14 [001] sugov_update_shared: this_cpu=1 sg_cpu->cpu=7
> sg_cpu->sg_policy->policy->related_cpus=4-7
> sugov:4-1492 [004] sugov_work: this_cpu=4
> sg_cpu->sg_policy->policy->related_cpus=4-7
> ...
>
> The remote callback (this_cpu=1, target_cpu=7) is executed on cpu=4.
>
> Now the sugov kthread executes again on the policy (cluster) for which
> the Operating Performance Point (OPP) should be changed.
> It avoids the problem that an otherwise idle policy (cluster) is running
> schedutil (the sugov kthread) for another one.
>
> Cc: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Pavan Kondeti <pkondeti@xxxxxxxxxxxxxx>
> Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
> Cc: Joel Fernandes <joelaf@xxxxxxxxxx>
> Cc: Patrick Bellasi <patrick.bellasi@xxxxxxx>
> Cc: Quentin Perret <quentin.perret@xxxxxxx>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> ---
> kernel/sched/cpufreq_schedutil.c | 6 +-----
> 1 file changed, 1 insertion(+), 5 deletions(-)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index d2c6083304b4..63014cff76a5 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -523,11 +523,7 @@ static int sugov_kthread_create(struct sugov_policy *sg_policy)
> }
>
> sg_policy->thread = thread;
> -
> - /* Kthread is bound to all CPUs by default */
> - if (!policy->dvfs_possible_from_any_cpu)
> - kthread_bind_mask(thread, policy->related_cpus);
> -
> + kthread_bind_mask(thread, policy->related_cpus);
> init_irq_work(&sg_policy->irq_work, sugov_irq_work);
> mutex_init(&sg_policy->work_lock);
>

Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>

--
viresh