linux-kernel - Re: [PATCH] cpufreq/schedutil: Only bind threads if needed

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Message-ID: <ZvPGacuDDXNa5k62@jlelli-thinkpadt14gen4.remote.csb>
Date: Wed, 25 Sep 2024 10:14:33 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Christian Loehle <christian.loehle@....com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-pm <linux-pm@...r.kernel.org>,
	Qais Yousef <qyousef@...alina.io>, Ingo Molnar <mingo@...hat.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Pierre Gondois <pierre.gondois@....com>
Subject: Re: [PATCH] cpufreq/schedutil: Only bind threads if needed

Hi,

On 12/09/24 17:01, Christian Loehle wrote:
> On 9/12/24 16:41, Rafael J. Wysocki wrote:
> > On Thu, Sep 12, 2024 at 3:53 PM Christian Loehle

...

> >> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> >> index c62acf509b74..7d4a4edfcfb9 100644
> >> --- a/kernel/sched/syscalls.c
> >> +++ b/kernel/sched/syscalls.c
> >> @@ -1159,6 +1159,9 @@ int dl_task_check_affinity(struct task_struct *p, const struct cpumask *mask)
> >>         if (!task_has_dl_policy(p) || !dl_bandwidth_enabled())
> >>                 return 0;
> >>
> >> +       if (dl_entity_is_special(&p->dl))
> >> +               return 0;
> >> +
> > 
> > Care to explain this particular piece?
> 
> Looks suspicious but the truncated comment below explains it:
> 	/*
> 	 * Since bandwidth control happens on root_domain basis,
> 	 * if admission test is enabled, we only admit -deadline
> 	 * tasks allowed to run on all the CPUs in the task's
> 	 * root_domain.
> 	 */
> So that would only allow setting it to all CPUs for the relevant
> platforms unfortunately.
> 
> That should be fine though since the sugov task is pretty much
> a dummy in terms of bandwidth / admission control internally, so
> no harm done to not enforce this when userspace wants to set
> affinities.
> ...Unless Juri disagrees.

Nope, I agree. :)

Wonder if we should put a comment along the lines of what you said above
right above the new conditions (so that people will not need to wonder
about it in the future). But not a strict requirement for me.

Thanks! (and apologies for the delay in replying)
Juri