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dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=efficios.com; Received: from YT6PR01MB491026.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:1d0::10) by YQ1PR01MB967408.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:b3::6) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.615.10; Fri, 13 Nov 2026 13:31:58 +0000 Received: from YT6PR01MB491026.CANPRD01.PROD.OUTLOOK.COM ([fe80::6b9e:a901:67c5:7ddc]) by YT6PR01MB491026.CANPRD01.PROD.OUTLOOK.COM ([fe80::6b9e:a901:67c5:7ddc%6]) with mapi id 15.21.0615.008; Fri, 13 Nov 2026 13:31:58 +0000 Message-ID: <3cb19d75-4269-467c-9d58-6c3ec26abc3f@efficios.com> Date: Fri, 13 Nov 2026 08:31:56 -0500 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 15/29] sched/deadline: Rebuild the root domains' accounting from the task list To: Juri Lelli Cc: peterz@infradead.org, mingo@redhat.com, vincent.guittot@linaro.org, linux-kernel@vger.kernel.org, tglx@kernel.org, will@kernel.org, boqun@kernel.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, kprateek.nayak@amd.com, luca.abeni@santannapisa.it, mathieu.poirier@linaro.org, longman@redhat.com, ridong.chen@linux.dev, tj@kernel.org, hannes@cmpxchg.org, mkoutny@suse.com, shuah@kernel.org, bigeasy@linutronix.de, clrkwllms@kernel.org, parri.andrea@gmail.com, m.szyprowski@samsung.com, kayracizmeci@gmail.com, weihengqiuu@gmail.com, cgroups@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-rt-devel@lists.linux.dev, stable@vger.kernel.org, Qais Yousef References: <20261108185406.1549770-1-mathieu.desnoyers@efficios.com> <20261108185406.1549770-16-mathieu.desnoyers@efficios.com> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: Content-Type: text/plain; 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You are right: the walk costs too much with no deadline task. I measured it. The test runs in a 64-vCPU guest with no debug options and no deadline task. A process holds N sleeping threads. A partition root is made and removed, and each change rebuilds the accounting. The time of one rebuild with this patch, in testing: threads one rebuild 0 0.04 to 0.08 ms 1000 0.12 to 0.17 ms 10000 0.7 to 1.2 ms 30000 2.3 ms The first three lines are from three runs, the last from one. Upstream returns early for a cpuset with no deadline task, so its rebuild does not grow with the number of threads. I did not measure it. From reading, a suspend runs one rebuild for each CPU it takes offline, and a resume one for each CPU it brings back. With 8 CPUs and 10000 threads that is 14 rebuilds: 10 to 16 ms on this host, and more on a phone. So the planned next version skips the walk when no task holds a deadline bandwidth, as you suggest. It keeps one count for the whole system, and none per cpuset: the number of tasks whose dl_bw is not 0. __setparam_dl() and __dl_clear_params() are the only places that change the dl_bw of a task, and they keep it. In the same test, a rebuild then takes 0.03 ms whatever the number of threads. With a deadline task in the system, the walk runs as before. A test on an Android device would still be welcome. For comparison, the single list of accounted tasks you suggested on patch 13 takes 0.03 ms in this test, whatever the number of threads. I left it out because the rebuild then runs with interrupts disabled for its whole length, which grows with the number of deadline tasks. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com