Re: [RFC] sched/eevdf: sched feature to dismiss lag on wakeup

From: Vincent Guittot <vincent.guittot@linaro.org>
To: Tobias Huschle <huschle@linux.ibm.com>
Cc: Luis Machado <luis.machado@arm.com>,
	linux-kernel@vger.kernel.org, mingo@redhat.com,
	 peterz@infradead.org, juri.lelli@redhat.com,
	dietmar.eggemann@arm.com,  rostedt@goodmis.org,
	bsegall@google.com, mgorman@suse.de, bristot@redhat.com,
	 vschneid@redhat.com, sshegde@linux.vnet.ibm.com,
	srikar@linux.vnet.ibm.com,  linuxppc-dev@lists.ozlabs.org,
	nd <nd@arm.com>
Subject: Re: [RFC] sched/eevdf: sched feature to dismiss lag on wakeup
Date: Wed, 20 Mar 2024 14:51:00 +0100	[thread overview]
Message-ID: <CAKfTPtBA7ECeYJYdzL9ybeXLbpEudLfB6V9s+DZiJUmpnPf_kQ@mail.gmail.com> (raw)
In-Reply-To: <65fa8a7c.050a0220.c8ec5.0278SMTPIN_ADDED_BROKEN@mx.google.com>

On Wed, 20 Mar 2024 at 08:04, Tobias Huschle <huschle@linux.ibm.com> wrote:
>
> On Tue, Mar 19, 2024 at 02:41:14PM +0100, Vincent Guittot wrote:
> > On Tue, 19 Mar 2024 at 10:08, Tobias Huschle <huschle@linux.ibm.com> wrote:
> > >
...
> > >
> > > Haven't seen that one yet. Unfortunately, it does not help to ignore the
> > > eligibility.
> > >
> > > I'm inclined to rather propose propose a documentation change, which
> > > describes that tasks should not rely on woken up tasks being scheduled
> > > immediately.
> >
> > Where do you see such an assumption ? Even before eevdf, there were
> > nothing that ensures such behavior. When using CFS (legacy or eevdf)
> > tasks, you can't know if the newly wakeup task will run 1st or not
> >
>
> There was no guarantee of course. place_entity was reducing the vruntime of
> woken up tasks though, giving it a slight boost, right?. For the scenario

It was rather the opposite, It was ensuring that long sleeping tasks
will not get too much bonus because of vruntime too far in the past.
This is similar although not exactly the same intent as the lag. The
bonus was up to 24ms previously whereas it's not more than a slice now

> that I observed, that boost was enough to make sure, that the woken up tasks
> gets scheduled consistently. This might still not be true for all scenarios,
> but in general EEVDF seems to be stricter with woken up tasks.
>
> Dismissing the lag on wakeup also does obviously not guarantee getting
> scheduled, as other tasks might still be involved.
>
> The question would be if it should be explicitly mentioned somewhere that,
> at this point, woken up tasks are not getting any special treatment and
> noone should rely on that boost for woken up tasks.
>
> > >
> > > Changing things in the code to address for the specific scenario I'm
> > > seeing seems to mostly create unwanted side effects and/or would require
> > > the definition of some magic cut-off values.
> > >
> > >