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[2.102.14.151]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-7077cde6062sm58919146d6.71.2025.08.04.07.49.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 04 Aug 2025 07:49:11 -0700 (PDT) Date: Mon, 4 Aug 2025 15:49:08 +0100 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: On 01/08/25 10:28, David Haufe wrote: > I am sorry, but we cannot get this branch to boot on our hardware. > Looking through the code of the branch, it will not address the issue. > I believe the issue is more fundamental. In > fair.c->enqueue_task_fair(), dl_server_start() is called when the > single fair/SCHED_OTHER task is added to the isolcpu/nohz_full core. > The check here is simply checking if there is 1 or more process and > kicks off the dl_server_start() and the housekeeping timer in > start_dl_timer(). Once this timer is running, it will invoke > dl_server_timer() continuously. This timer calls __enqueue_dl_entity() > and then inc_dl_tasks(). inc_dl_tasks() increments > dl_rq->dl_nr_running++ and invokes add_nr_running(). This code will > eventually call the sched_can_stop_tick() function but > rq->dl.dl_nr_running now != 0, so this function will always return > false. Something needs to be done to prevent this timer from running > in the first place, or maybe have some checks around single > "fair/SCHED_OTHER/etc" process running on an isolcpu/nohz_full core > which prevents the need for the deadline code to run for the core. The fix commit I mentioned should at least make entering nohz_full work again even when the dl_server is active (but deferred). We still have the 1 dl_server_timer firing each second (after recent additional fix by Peter), though. At least this is what I am seeing at my end. Will try to see if we can remove that periodic timer once nohz_full mode is entered. Thanks, Juri