linux-kernel - Re: [PATCH] sched/deadline: Add DL server activated message

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Message-ID: <9f91f77e5f39857aa84373fe1ae504de2a881533.camel@mediatek.com>
Date: Thu, 7 Aug 2025 15:52:23 +0800
From: Kuyo Chang <kuyo.chang@...iatek.com>
To: Juri Lelli <juri.lelli@...hat.com>
CC: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann
	<dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, "Ben
 Segall" <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, "Valentin
 Schneider" <vschneid@...hat.com>, Matthias Brugger <matthias.bgg@...il.com>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	<linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-mediatek@...ts.infradead.org>
Subject: Re: [PATCH] sched/deadline: Add DL server activated message

On Thu, 2025-08-07 at 08:15 +0100, Juri Lelli wrote:
> 
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
> 
> 
> Hi,
> 
> On 05/08/25 23:53, Kuyo Chang wrote:
> > From: kuyo chang <kuyo.chang@...iatek.com>
> > 
> > The DL server is introduced as a replacement for realtime
> > throttling.
> > When RT throttling is activated, a message
> > "sched: RT throttling activated" is shown. However, it is currently
> > difficult for users to know when the DL server is activated.
> > 
> > This patch adds a similar message to indicate when the DL server
> > is activated, which helps users debug RT/CFS contention issues.
> > 
> > Signed-off-by: kuyo chang <kuyo.chang@...iatek.com>
> > ---
> >  kernel/sched/deadline.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index e2d51f4306b3..8e0de6cdb980 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -2042,6 +2042,9 @@ enqueue_dl_entity(struct sched_dl_entity
> > *dl_se, int flags)
> >       }
> > 
> >       __enqueue_dl_entity(dl_se);
> > +
> > +     if (dl_server(dl_se))
> > +             printk_deferred_once("sched: dl_server activated\n");
> >  }
> 
> Not sure if we want/need this, but, if we do, I believe
> fair_server_pick_task() might be a better place to put it, as it's
> really when the dl-server is called to do its job.
> 

Thanks for your suggestion, maybe the minor patch as below

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index e2d51f4306b3..82d1091e56f5 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2395,6 +2395,7 @@ static struct task_struct *__pick_task_dl(struct
rq *rq)
 			goto again;
 		}
 		rq->dl_server = dl_se;
+		printk_deferred_once("sched: dl_server activated\n");
 	} else {
 		p = dl_task_of(dl_se);
 	}
-- 

I believe the debug message could help to verify that the DL server is
actually working.
It will be a great help for users to debug RT/FAIR contention issues
when this clue message occurs.

> Thanks,
> Juri
>