linux-kernel - Re: [RFC PATCH] sched/feec: Simplify the traversal of pd'cpus

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Message-ID: <CAB8ipk8W8Oj2vu14sGX0wncPpJRD=xL35U8QW1wou-CX+3H0KA@mail.gmail.com>
Date: Tue, 19 Aug 2025 10:02:52 +0800
From: Xuewen Yan <xuewen.yan94@...il.com>
To: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Christian Loehle <christian.loehle@....com>, Xuewen Yan <xuewen.yan@...soc.com>, mingo@...hat.com, 
	peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org, 
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, 
	vdonnefort@...gle.com, ke.wang@...soc.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] sched/feec: Simplify the traversal of pd'cpus

On Mon, Aug 18, 2025 at 11:24 PM Dietmar Eggemann
<dietmar.eggemann@....com> wrote:
>
> On 18.08.25 12:05, Xuewen Yan wrote:
> > On Fri, Aug 15, 2025 at 9:01 PM Dietmar Eggemann
> > <dietmar.eggemann@....com> wrote:
> >>
> >> On 14.08.25 10:52, Xuewen Yan wrote:
> >>> Hi Dietmar,
> >>>
> >>> On Thu, Aug 14, 2025 at 4:46 PM Dietmar Eggemann
> >>> <dietmar.eggemann@....com> wrote:
> >>>>
> >>>> On 12.08.25 10:33, Xuewen Yan wrote:
> >>
> >> [...]
> >>
> >>>> Can you not mask cpus already early in the pd loop (1) and then profit
> >>>> from (2) in these rare cases?
> >>>
> >>> I do not think the cpus_ptr chould place before the pd_cap calc,
> >>> because the following scenario should be considered:
> >>> the task's cpus_ptr cpus： 0,1,2,3
> >>> pd's cpus: 0,1,2,3,4,5,6
> >>> the pd's cap = cpu_cap * 6;
> >>> if we cpumask_and(pd'scpus, p->cpus_ptr),
> >>> the cpumask_weight = 4,
> >>> the pd's cap = cpu_cap *4.
> >>
> >> Yes, you're right! Missed this one.
> >>
> >>>> IIRC, the sd only plays a role here in
> >>>> exclusive cpusets scenarios which I don't thing anybody deploys with EAS?
> >>>
> >>> I am also wondering if the check for SD's CPUs could be removed...
> >>
> >> Still not 100% sure here. I would have to play with cpusets and EAS a
> >> little bit more. Are you thinking that in those cases p->cpus_ptr
> >> already covers the cpuset restriction so that the sd mask isn't necessary?
> >
> > I am not familiar with cpuset, so I can't guarantee this. Similarly, I
> > also need to learn more about cpuset and cpu topology before I can
> > answer this question.
>
> Looks like we do need also the sd cpumask here.
>
> Consider this tri-gear system:
>
> #  cat /sys/devices/system/cpu/cpu*/cpu_capacity
> 160
> 160
> 160
> 160
> 498
> 498
> 1024
> 1024
>
> and 2 exclusive cpusets cs1={0-1,4,6} and cs2={2-3,5,7}, so EAS is
> possible in all 3 root_domains (/, /cs1, /cs2):

Isn't your CPU an ARM Dynamiq architecture?
In my understanding, for Dynamiq arch, there is only one MC domain...
Did I miss something?

Thanks!
>
> ...
> [   74.691104] CPU1 attaching sched-domain(s):
> [   74.691180]  domain-0: span=0-1 level=MC
> [   74.691244]   groups: 1:{ span=1 cap=159 }, 0:{ span=0 cap=155 }
> [   74.693453]   domain-1: span=0-1,4,6 level=PKG
> [   74.693534]    groups: 0:{ span=0-1 cap=314 }, 4:{ span=4 cap=496 },
> 6:{ span=6 cap=986 }
> ...
> [   74.697890] root domain span: 0-1,4,6
> [   74.697994] root_domain 2-3,5,7: pd6:{ cpus=6-7 nr_pstate=4 } pd4:{
> cpus=4-5 nr_pstate=4 } pd0:{ cpus=0-3 nr_pstate=4 }
> [   74.698922] root_domain 0-1,4,6: pd6:{ cpus=6-7 nr_pstate=4 } pd4:{
> cpus=4-5 nr_pstate=4 } pd0:{ cpus=0-3 nr_pstate=4 }
>
>
>   sd = rcu_dereference(*this_cpu_ptr(&sd_asym_cpucapacity));
>
>
> Tasks running in '/' only have the sd to reduce the CPU affinity correctly.
>
> ...
> [001] 5290.935663: select_task_rq_fair: kworker/u33:3 382 prev_cpu=0
> [001] 5290.935696: select_task_rq_fair: kworker/u33:3 382 prev_cpu=0
> pd=6-7 online=0-7 sd=0-1,4,6 cpus_ptr=0-7
> [001] 5290.935753: select_task_rq_fair: kworker/u33:3 382 prev_cpu=0
> pd=4-5 online=0-7 sd=0-1,4,6 cpus_ptr=0-7
> [001] 5290.935779: select_task_rq_fair: kworker/u33:3 382 prev_cpu=0
> pd=0-3 online=0-7 sd=0-1,4,6 cpus_ptr=0-7
> ...