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Thu, 31 Jul 2025 00:01:55 -0700 (PDT) X-Google-Smtp-Source: AGHT+IF3Nf3k6F37Mrq3YoJxogERWli6tP0u7To4PZTrfP7eJEm+0ri9H5Ynf4uVNuH5XKqRPejdDg== X-Received: by 2002:a05:6000:430b:b0:3b4:9ade:4e8a with SMTP id ffacd0b85a97d-3b79d4e0b4bmr706055f8f.21.1753945314919; Thu, 31 Jul 2025 00:01:54 -0700 (PDT) Received: from jlelli-thinkpadt14gen4.remote.csb ([151.57.5.154]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b79c3ac574sm1334218f8f.5.2025.07.31.00.01.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 31 Jul 2025 00:01:54 -0700 (PDT) Date: Thu, 31 Jul 2025 09:01:21 +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=us-ascii Content-Disposition: inline In-Reply-To: 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? Thanks, Juri