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Fri, 01 Aug 2025 02:06:09 -0700 (PDT) X-Google-Smtp-Source: AGHT+IGTi1Fa9TEH8nqKMEd+slRdcxzYt6awfrK20vVt4LCyIMlXwe2dJepCulZgCynDTlUyIzj2Eg== X-Received: by 2002:a05:6000:24c6:b0:3b7:6828:5f78 with SMTP id ffacd0b85a97d-3b8d343afadmr1682683f8f.4.1754039168597; Fri, 01 Aug 2025 02:06:08 -0700 (PDT) Received: from jlelli-thinkpadt14gen4.remote.csb ([151.29.46.230]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b79c4a6ff7sm5236741f8f.75.2025.08.01.02.06.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 01 Aug 2025 02:06:07 -0700 (PDT) Date: Fri, 1 Aug 2025 11:06:06 +0200 From: Juri Lelli To: David Haufe Cc: linux-kernel@vger.kernel.org Subject: Re: Kernel 6.14.11 dl_server_timer(...) causing IPI/Function Call Interrupts on isolcpu/nohz_full cores, performance regression Message-ID: References: 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=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Hi, On 31/07/25 12:48, David Haufe wrote: > Kernel 6.16 shows the issue. /kernel/sched/fair.c calls dl_server_start() > and there is no assessment prior to that point or later of the > isolcpu/nohz_full+single-process condition of the core. Same function_graph > trace generated. Code is the same at tip+sched/core. > > On Thu, Jul 31, 2025 at 2:02 AM Juri Lelli wrote: > > > Hello, > > > > Thanks for the report. > > > > On 30/07/25 11:51, David Haufe wrote: > > > [1.] Kernel 6.14.11 dl_server_timer(...) causing IPI/Function Call > > > Interrupts on isolcpu/nohz_full cores, performance regression > > > [2.] The code for dl_server_timer is causing new IPI/Function Call > > > Interrupts to fire on isolcpu/nohz_full cores which previously had no > > > interrupts. When there is a single, SCHED_OTHER process running on an > > > isolcpu/nohz_full core, dl_server_timer executes on a housekeeping > > > core. This ultimately invokes add_nr_running() and > > > sched_update_tick_dependency() and finally tick_nohz_dep_set_cpu(). > > > Setting the single process running on an isolcpu/nohz_full core to > > > FIFO (rt priority) prevents this new interrupt, as it is not seen as a > > > fair schedule process anymore. Having to use rt priority is > > > unnecessary and a regression to prior kernels. Kernel function_graph > > > trace below showing core 0 (housekeeping) sending the IPI to core 19 > > > (nohz_full, isolcpu, rcu_nocb_poll) which is running a single > > > SCHED_OTHER process. I believe this has been observed by others. > > > > > https://community.clearlinux.org/t/sysjitter-worse-in-kernel-6-12-than-6-6/10206 > > > > Would you be able to check if the following branch, containing multiple > > fixes for dl-server, is still affected by the regression? Apologies, I forgot to share the actual branch. :-/ Could you please test with https://github.com/jlelli/linux/commits/upstream/fix-dlserver-1/ Among various other fixes, 219a63335b67 ("sched/deadline: Don't count nr_running twice for dl_server proxy tasks") is making sure we don't count fair tasks twice, so I am wondering if it can have an effect on entering nohz_full. Thanks, Juri