linux-kernel - Re: [PATCH] sched/deadline: Derive root domain from active cpu in task's cpus

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Message-ID: <93fab42a-c9d7-4660-98f8-38e6f532182c@arm.com>
Date: Mon, 6 Oct 2025 12:20:13 +0200
From: Pierre Gondois <pierre.gondois@....com>
To: Pingfan Liu <piliu@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Juri Lelli <juri.lelli@...hat.com>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH] sched/deadline: Derive root domain from active cpu in
 task's cpus_ptr


On 9/30/25 11:04, Peter Zijlstra wrote:
> On Tue, Sep 30, 2025 at 08:20:06AM +0100, Juri Lelli wrote:
>
>> I actually wonder if we shouldn't make cppc_fie a "special" DEADLINE
>> tasks (like schedutil [1]). IIUC that is how it is thought to behave
>> already [2], but, since it's missing the SCHED_FLAG_SUGOV flag(/hack),
>> it is not "transparent" from a bandwidth tracking point of view.
>>
>> 1 - https://elixir.bootlin.com/linux/v6.17/source/kernel/sched/cpufreq_schedutil.c#L661
>> 2 - https://elixir.bootlin.com/linux/v6.17/source/drivers/cpufreq/cppc_cpufreq.c#L198
> Right, I remember that hack. Bit sad its spreading, but this CPPC thing
> is very much like the schedutil one, so might as well do that I suppose.
>
IIUC, the sugov thread was switched to deadline to allow frequency updates
when deadline tasks start to run. I.e. there should be no point updating 
the
freq. after the deadline task finished running, cf [1] and [2]

The CPPC FIE worker should not require to run that quickly as it seems to be
more like a freq. maintenance work (the call comes from the sched tick)

sched_tick()
\-arch_scale_freq_tick() / topology_scale_freq_tick()
   \-set_freq_scale() / cppc_scale_freq_tick()
     \-irq_work_queue()


[1] https://lore.kernel.org/all/20171204102325.5110-3-juri.lelli@redhat.com/
[2] https://lore.kernel.org/all/20171204102325.5110-1-juri.lelli@redhat.com/
"""
  o 03/08      it's a temporary solution to make possible (on ARM) to change

               frequency for DEADLINE tasks (that would possibly delay 
the SCHED_FIFO
               worker kthread); proper solution would be to be able to 
issue frequency
               transition from an atomic ctx
"""