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Tue, 12 May 2026 08:31:47 -0700 (PDT) Received: from jlelli-thinkpadt14gen4.remote.csb ([151.29.56.132]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48e8e60620bsm21209855e9.6.2026.05.12.08.31.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 12 May 2026 08:31:47 -0700 (PDT) Date: Tue, 12 May 2026 17:31:45 +0200 From: Juri Lelli To: Frederic Weisbecker Cc: Ingo Molnar , Peter Zijlstra , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , K Prateek Nayak , Andrea Righi , linux-kernel@vger.kernel.org, David Haufe , Cao Ruichuang , Tejun Heo Subject: Re: [PATCH] sched/deadline: Make dl-server nohz full aware Message-ID: References: <20260512-upstream-fix-dlserver-nohzfull-b4-v1-1-a94844387ae7@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On 12/05/26 16:03, Frederic Weisbecker wrote: > Le Tue, May 12, 2026 at 11:02:37AM +0200, Juri Lelli a écrit : > > The dl_server_timer() causes spurious IPIs on nohz_full cores, breaking > > isolation guarantees. The timer executes on a housekeeping core and > > eventually calls tick_nohz_dep_set_cpu(), sending IPIs to isolated cores > > even when only a single task is running. > > > > The problem is that dl-servers are not coordinated with nohz_full tick > > state. Timers can fire and send IPIs to otherwise undisturbed cores. > > > > Fix by managing servers in sched_can_stop_tick(): > > > > - When RT tasks run with CFS/SCX tasks, start the appropriate server > > and keep the tick running > > - When only RT tasks remain, stop all servers and allow tick to stop > > (except for >1 RR tasks which need the tick for round-robin) > > - When only CFS/SCX tasks remain, stop all servers before stopping tick > > > > Introduce dl_servers_stop_all() to reduce duplication and abstract > > server management from core.c. Unify RT handling into one block that > > handles both RR and FIFO cases. > > > > Fixes: 557a6bfc662c ("sched/fair: Add trivial fair server") > > Reported-by: David Haufe > > Closes: https://lore.kernel.org/lkml/CAKJHwtOw_G67edzuHVtL1xC5Vyt6StcZzihtDd0yaKudW=rwVw@mail.gmail.com > > Signed-off-by: Juri Lelli > > I indeed observed IPIs originating from dl_server some time > ago but that magically disappeared after some commit from Peter. > > Perhaps it came back somehow? Lemme run dynticks-testing again on > latest upstream... So, the IPIs seem to be indeed gone also from my recent testing (decided to keep the reference/context in the changelog for historical reasons, but can remove). But the dl_task_timer (dl-server timer) for isolated CPUs is still firing on housekeeping and that is what this is addressing. > > --- > > I had to modify my first original attempt at fixing this (please take a > > look at the linked report/discussion) to also take SCX into > > consideration. > > I thought SCX was disabled when CPU isolation is running? > > 9f391f94a173 ("sched_ext: Disallow loading BPF scheduler if isolcpus= domain > isolation is in effect") Ah, thanks for poiting it out, it indeed simplify things. :) And thanks for the super quick review! Best, Juri