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Date: Thu, 1 Oct 2026 17:33:01 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Juri Lelli <juri.lelli@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Valentin Schneider <vschneid@redhat.com>,
	K Prateek Nayak <kprateek.nayak@amd.com>,
	Andrea Righi <arighi@nvidia.com>,
	Frederic Weisbecker <frederic@kernel.org>,
	linux-kernel@vger.kernel.org,
	David Haufe <dhaufe@simplextrading.com>,
	Cao Ruichuang <create0818@163.com>,
	Furkan =?utf-8?B?w4dhbMSxxZ9rYW4=?= <frn1furkan10@gmail.com>,
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Subject: Re: [PATCH v2] sched/deadline: Make dl-server nohz full aware
Message-ID: <20261001153301.GS88198@noisy.programming.kicks-ass.net>
References: <20260513-upstream-fix-dlserver-nohzfull-b4-v2-1-d3e9cbe5c845@redhat.com>
 <20261001142735.GT2009045@noisy.programming.kicks-ass.net>
 <20261001143314.GK4121620@noisy.programming.kicks-ass.net>
 <ar57h-DsEVtuZnVw@jlelli-thinkpadt14gen4.remote.csb>
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In-Reply-To: <ar57h-DsEVtuZnVw@jlelli-thinkpadt14gen4.remote.csb>

On Thu, Oct 01, 2026 at 05:25:59PM +0200, Juri Lelli wrote:
> On 01/10/26 16:33, Peter Zijlstra wrote:
> > On Thu, Oct 01, 2026 at 04:27:36PM +0200, Peter Zijlstra wrote:
> > 
> > > >  bool sched_can_stop_tick(struct rq *rq)
> > > >  {
> > > >  	/* Deadline tasks, even if single, need the tick */
> > > >  	if (rq->dl.dl_nr_running)
> > > >  		return false;
> > > >  
> > > >  	/*
> > > > +	 * If there are RT tasks, we may need the tick (for >1 RR tasks),
> > > > +	 * but we must also service lower-priority CFS/SCX tasks via dl-servers.
> > > >  	 */
> > > > +	if (rq->rt.rt_nr_running) {
> > > > +		bool cfs_or_scx_queued = false;
> > > > +
> > > > +		if (rq->cfs.h_nr_queued) {
> > > > +			dl_server_start(&rq->fair_server);
> > > > +			cfs_or_scx_queued = true;
> > > > +		}
> > > > +#ifdef CONFIG_SCHED_CLASS_EXT
> > > > +		if (rq->scx.nr_running) {
> > > > +			dl_server_start(&rq->ext_server);
> > > > +			cfs_or_scx_queued = true;
> > > > +		}
> > > > +#endif
> > > > +		if (cfs_or_scx_queued)
> > > >  			return false;
> > 
> > What the purpose of this boolean, both true assignments above can simply
> > be return false, no?
> 
> But we won't start the ext_server, even if EXT tasks are present, if we
> return false in case CFS tasks are present.

Ah, see, I need a break ;-)

/me stomps off to brew tea...