linux-kernel - Re: [PATCH] sched/pi: Reweight fair_policy() tasks when inheriting prio

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Date: Mon, 8 Apr 2024 09:16:52 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Qais Yousef <qyousef@...alina.io>
Cc: Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Daniel Bristot de Oliveira <bristot@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Joel Fernandes <joel@...lfernandes.org>, 
	John Stultz <jstultz@...gle.com>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	linux-kernel@...r.kernel.org, Yabin Cui <yabinc@...gle.com>
Subject: Re: [PATCH] sched/pi: Reweight fair_policy() tasks when inheriting prio

On Sun, 7 Apr 2024 at 14:27, Qais Yousef <qyousef@...alina.io> wrote:
>
> On 04/05/24 18:16, Qais Yousef wrote:
>
> > >
> > > All that to say that I think the weight is not applied on purpose.
> > > This might work for your particular case but there are more changes to
> > > be done if you want to apply prio inheritance between cfs tasks.
> > >
> > > As an example, what about the impact of cgroup on the actual weight
> > > and the inherited priority of a task ? If the owner and the waiter
> > > don't belong to the same cgroup their own prio is meaningless... task
> > > nice -20 in a group with a weight equal to nice 19 vs a task nice 19
> > > in a group with a weight equals to nice -20
> >
> > That is on my mind actually. But I thought it's a separate problem. That has to
> > do with how we calculate the effective priority of the pi_task. And probably
> > the sorting order to if we agree we need to revert the above. If that is done
>
> Thinking more about it the revert is not the right thing to do. We want fair
> tasks to stay ordered in FIFO for better fairness and avoid potential
> starvation issues. It's just the logic for searching the top_waiter need to be
> different. If the top_waiter is fair, then we need to traverse the tree to find
> the highest nice value. We probably can keep track of this while adding items
> to the tree to avoid the search.
>
> For cgroup; is it reasonable (loosely speaking) to keep track of pi_cfs_rq and
> detach_attach_task_cfs_rq() before the reweight? This seems the most
> straightforward solution and will contain the complexity to keeping track of
> cfs_rq. But it'll have similar issue to proxy execution where a task that
> doesn't belong to the cgroup will consume its share..

That's a good point, Would proxy execution be the simplest way to fix all this ?

>
> Can we treat the two as separate problems? Or you think any solution must
> address the two? Both must be fixed of course.
>
>
> Thanks!
>
> --
> Qais Yousef