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Tue, 03 Nov 2026 15:34:11 -0800 (PST) Received: from fedora ([2.35.201.240]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4b43c17ecb4sm9524955e9.3.2026.11.03.15.34.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 03 Nov 2026 15:34:10 -0800 (PST) From: Francesco Nigro To: Mike Galbraith Cc: Ingo Molnar , Peter Zijlstra , Vincent Guittot , Dietmar Eggemann , Valentin Schneider , K Prateek Nayak , Mario Limonciello , Huang Rui , linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org Subject: Re: [REPORT] sched: ITMT + DELAY_DEQUEUE: woken task waits ~1 ms behind another task on the preferred CPU while allowed CPUs are idle Date: Wed, 4 Nov 2026 00:33:52 +0100 Message-ID: <20261103233352.2282883-1-nigro.fra@gmail.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <25edb6cd36cb77b82b40826b97232438f1443516.camel@gmx.de> References: <20261103154603.1893325-1-nigro.fra@gmail.com> <25edb6cd36cb77b82b40826b97232438f1443516.camel@gmx.de> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Tue, 2026-11-03 at 18:27 +0100, Mike Galbraith wrote: > Mmm, that's a be careful what you wish for thing. [...] > Not a keeper. Thanks, that answers my second question for your patch; I had listed it as earlier work on the same step. The stalls in the report happen when both busy threads are on CPU 6 at the same time. What the report doesn't show is where the two threads run the rest of the time. With ITMT on, both run almost only on CPUs 6 and 7, the two highest-ranked of the four: each starts about half of its bursts on each, and under 3 % on CPUs 4 and 5. With ITMT off, the two threads' main CPUs differ in every run (run 1: one thread starts 99.9 % of its bursts on CPU 5, the other 100 % on CPU 6). Per-CPU burst counts from the earlier reproducer (asym_stack2.c), 5 runs of 30 s: https://github.com/franz1981/itmt-delay-dequeue-repro/blob/49b6d72cb41d2535964c87a660178dd636d1d366/results-final.txt Is it expected that ITMT keeps two busy threads on the same two top-ranked CPUs, sometimes on the same one, while the other allowed CPUs are idle? Thanks, Franz