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[89.240.117.139]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6e8f715b5f9sm5383796d6.81.2025.03.06.01.35.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 06 Mar 2025 01:35:31 -0800 (PST) Date: Thu, 6 Mar 2025 09:35:27 +0000 From: Juri Lelli To: Joel Fernandes Cc: Ingo Molnar , Peter Zijlstra , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , stable@vger.kernel.org, "Paul E . McKenney" , linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] sched/fair: Disable DL server on rcu_torture_disable_rt_throttle() Message-ID: References: <20250306011014.2926917-1-joelagnelf@nvidia.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=us-ascii Content-Disposition: inline In-Reply-To: <20250306011014.2926917-1-joelagnelf@nvidia.com> Hi Joel, On 05/03/25 20:10, Joel Fernandes wrote: > Currently, RCU boost testing in rcutorture is broken because it relies on > having RT throttling disabled. This means the test will always pass (or > rarely fail). This occurs because recently, RT throttling was replaced > by DL server which boosts CFS tasks even when rcutorture tried to > disable throttling (see rcu_torture_disable_rt_throttle()). However, the > systctl_sched_rt_runtime variable is not considered thus still allowing > RT tasks to be preempted by CFS tasks. > > Therefore this patch prevents DL server from starting when RCU torture > sets the sysctl_sched_rt_runtime to -1. > > With this patch, boosting in TREE09 fails reliably if RCU_BOOST=n. > > Steven also mentioned that this could fix RT usecases where users do not > want DL server to be interfering. > > Cc: stable@vger.kernel.org > Cc: Paul E. McKenney > Cc: Steven Rostedt > Fixes: cea5a3472ac4 ("sched/fair: Cleanup fair_server") > Signed-off-by: Joel Fernandes > --- > v1->v2: > Updated Fixes tag (Steven) > Moved the stoppage of DL server to fair (Juri) I think what I suggested/wondered (sorry if I wasn't clear) is that we might need a link between sched_rt_runtime and the fair_server per-cpu runtime under sched/debug (i.e., sched_fair_write(), etc), otherwise one can end up with DL server disabled and still non zero runtime on the debug interface. This is only if we want to make that link, though; which I am not entirely sure it is something we want to do, as we will be stuck with an old/legacy interface if we do. Peter? Thanks, Juri