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Fri, 06 Nov 2026 18:15:13 -0800 (PST) Received: from localhost.localdomain ([86.124.84.219]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-492d7cc5f11sm6367449f8f.49.2026.11.06.18.15.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 06 Nov 2026 18:15:13 -0800 (PST) From: Radu Toader To: Tejun Heo Cc: peterz@infradead.org, mingo@redhat.com, tglx@kernel.org, void@manifault.com, arighi@nvidia.com, changwoo@igalia.com, linux-kernel@vger.kernel.org, sched-ext@lists.linux.dev, bpf@vger.kernel.org Subject: Re: [PATCH 1/2] sched_ext: Add scx_tickbug, a scheduler that holds a task on an idle CPU Date: Sat, 7 Nov 2026 02:15:06 +0000 Message-ID: <20261107021509.968928-1-radu.m.toader@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: References: <20261106094343.174905-1-radu.m.toader@gmail.com> <20261106094343.174905-2-radu.m.toader@gmail.com> 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 Fri, Nov 06, 2026 at 06:59:50AM -1000, Tejun Heo wrote: > Thanks for catching the bug. However, this scheduler is more of an one-off > debug code and I'm not sure we want to carry this in tree. Maybe put it in a > github gist or sth and point to it from the fix patch? Fair enough. v2 is the one-line fix on its own. Rather than a gist, the fix points at the v1 posting of this patch in the archive, so the link cannot rot and needs no account: https://lore.kernel.org/lkml/20261106094343.174905-2-radu.m.toader@gmail.com/ One offer before I drop it entirely. tools/testing/selftests/sched_ext already carries nohz_tick, which asserts that a finite-slice EXT task restarts the scheduler tick after an idle interval on a NOHZ_FULL CPU. The condition this scheduler holds could be asserted there as a selftest instead of carried as an example scheduler in tools/sched_ext. If a regression guard for this is worth having I will post it separately; if not, the archive link is enough and I will leave it alone. Either way, the reproducer turned out to have a bug of its own, which the CI review found on the same posting: an untagged task pinned to the target CPU starved in the shared DSQ, because the target CPU stops consuming that DSQ while it is holding something and no other CPU may take a task pinned there. With two tagged tasks handing off it could sit until the watchdog unloaded the scheduler, which contradicts the "the machine stays usable" claim in the commit message. Dispatching anything with nr_cpus_allowed == 1 to its local DSQ fixes it. Noting it here in case someone takes the thing out of the archive. Thanks for the quick look. Disclosure: this reply was written by Claude Opus 5 (Claude Code) and sent on my behalf, in the same session as the patch; the patch changelog carries the same note.