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

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Message-ID: <20251030184205.GB2989771@noisy.programming.kicks-ass.net>
Date: Thu, 30 Oct 2025 19:42:05 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: Juri Lelli <juri.lelli@...hat.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Clark Williams <williams@...hat.com>, arighi@...dia.com
Subject: Re: [RFC PATCH] sched/deadline: Avoid dl_server boosting with
 expired deadline

On Wed, Oct 22, 2025 at 12:11:51PM +0200, Gabriele Monaco wrote:

Sorry, finally cycling back to this.

> > So how about something like this for starters?
> > 
> 
> Thanks Peter for sharing this patch, I run it through my test and the model
> seems to pass (i.e. no more boosting after deadline). What I found curious
> however, is that throughout the test, servers went only through replenish
> events.
> The system under test is mostly idle (6 periodic dl tasks on a 16 CPUs virtme-ng
> VM), so I expect not to see any task boosted by the servers, but in 5 minutes I
> didn't even observe any start/stop for the server.
> 
> I'm not sure why this is happening, but looking at traces it seems replenish
> occurs more often and perhaps doesn't let the server stop:
> 
> <idle>-0     [009] d.h3.    14.312395: (+950124) event_nomiss:         -9: idle x dl_replenish_idle -> idle
> <idle>-0     [009] d.h3.    14.312401: (+6)     sched_dl_replenish:   comm=server pid=-9 runtime=50000000 deadline=15253307235 yielded=0
> <idle>-0     [009] d.h3.    15.262771: (+950370) event_nomiss:         -9: idle x dl_replenish_idle -> idle
> <idle>-0     [009] d.h3.    15.262781: (+10)    sched_dl_replenish:   comm=server pid=-9 runtime=50000000 deadline=16203668554 yielded=0
> <idle>-0     [009] d.h3.    16.213117: (+950336) event_nomiss:         -9: idle x dl_replenish_idle -> idle
> <idle>-0     [009] d.h3.    16.213123: (+6)     sched_dl_replenish:   comm=server pid=-9 runtime=50000000 deadline=17154029879 yielded=0
> 
> Is this expected?

Sort of, that was next on the list. Let me see if I can make it stop a
little more.