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

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Message-ID: <aOOyF3EvIG5HKEel@jlelli-thinkpadt14gen4.remote.csb>
Date: Mon, 6 Oct 2025 14:12:07 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Pierre Gondois <pierre.gondois@....com>
Cc: Pingfan Liu <piliu@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	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 06/10/25 12:13, Pierre Gondois wrote:
> 
> 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()

OK, but how much bandwidth is enough for it (on different platforms)?
Also, I am not sure the worker follows cpusets/root domain changes.