linux-kernel - Re: [RFC PATCH] sched/deadline: Avoid dl_server boosting with expired deadline

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Message-ID: <a0ccf27f5e12a11d2e9dc951ceaf7f9d103f67f6.camel@redhat.com>
Date: Tue, 14 Oct 2025 12:05:06 +0200
From: Gabriele Monaco <gmonaco@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, Juri Lelli <juri.lelli@...hat.com>, Ingo
 Molnar <mingo@...hat.com>, Clark Williams <williams@...hat.com>
Subject: Re: [RFC PATCH] sched/deadline: Avoid dl_server boosting with
 expired deadline

On Tue, 2025-10-14 at 11:54 +0200, Peter Zijlstra wrote:
> On Tue, Oct 07, 2025 at 02:29:04PM +0200, Gabriele Monaco wrote:
> > Recent changes to the deadline server leave it running when the system
> > is idle. If the system is idle for longer than the dl_server period and
> > the first scheduling occurs after a fair task wakes up, the algorithm
> > picks the server as the earliest deadline (in the past) and that boosts
> > the fair task that just woke up while:
> >  * the deadline is in the past
> >  * the server consumed all its runtime (in background)
> >  * there is no starvation (idle for about a period)
> > 
> > Prevent the server from boosting a task when the deadline is in the
> > past. Instead, replenish a new period and start the server as deferred.
> 
> I'm a bit confused, should not enqueue ensure deadline is in the future?
> And if it doesn't shouldn't we fix the enqueue path somewhere?

Enqueue of a deadline task should handle the case, here the CPU is idle and the
deadline server did not stop yet (and won't until the next schedule, if I'm not
mistaken).
The following enqueue of a fair task triggers a schedule where the server (no
longer deferred) boosts the task straight away.

Now the only check for deadline is in pick_next_dl_entity, where the earliest
one is chosen, despite being in the past.

Do you mean to check for deadline when enqueueing the fair task too? I believe
again nothing happens here because the server is still up.

Does it make sense or am I missing something?