From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-ej1-f71.google.com (mail-ej1-f71.google.com [209.85.218.71]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A4342331EAE for ; Mon, 19 Oct 2026 12:41:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.218.71 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792413720; cv=none; b=kum5z+jHALTLfzEsR7pIhiewoWZgjfrYTvidLbfqvff/49EG2rV2rJwWe4ACeniYw+MJ7IJvjxvfJ9O/vXvCfjwYUBLlBG0AtGCF4Vdzhhp/rHCPIU0uj21CkpeZE4digP0gmmtj75XQ/kpCrlvyd1/sJEDNzVgguziP1glvJ4g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792413720; c=relaxed/simple; bh=gHiozBA4b+Mydjj0WDx+4yc4fZiEG1xZdsLUi/YhAe4=; h=Date:Mime-Version:Message-ID:Subject:From:To:Cc:Content-Type; b=nvo4/v5yCJ32zBe5D0BK9Ne8COQOY2Q+btZ/2YR4MQBSAUDWggn32m7f2YqeONTOvkGle6BI/jgaXEg6PAb/Zcy0p5g1S7TdJCLMEAuc3eWYggtYFR4GsMGaoMcIb9rrhlZw8VitAqeiSDGwxH5WJUKNKv+z8yN9rw1KqLpFkeM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--michalblk.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=ubLcBqq9; arc=none smtp.client-ip=209.85.218.71 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--michalblk.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="ubLcBqq9" Received: by mail-ej1-f71.google.com with SMTP id a640c23a62f3a-c41dae9db9dso163070566b.2 for ; Mon, 19 Oct 2026 05:41:51 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1792413709; x=1793018509; darn=vger.kernel.org; h=content-type:cc:to:from:subject:message-id:mime-version:date:from :to:cc:subject:date:message-id:reply-to:content-type; bh=xG7Y20y+nUaq6lJVPnnQYX7VFSFS/lNQS0CTHINS8qU=; b=ubLcBqq9r9QYqQnEApsKR8BOdHC0ijuPA5UQpV1wG5xoiIhjfyyzBhfSXv14cXNQGn 46S/FO8Hu3IJhucT7TRVXITAGZzI+SeaW+jeAyZA3xws1qsonQvcti/aQ7J3Kh2waXaF j6OVlla0oxtyLEonI2SgnyIVfUV8NneuF86H7YIP8dczPGdtMPSTBuHsEpyPPYjCdvoc YrUcXn3Ru+gwuPVg3/o52uakrv7brduJ2vPbKjxQjPdyGxmk7lwbwCJiguWSGn3KAZki HyqrgxnqC00nGHYcpXch17+UnzPQ3vM9QhDnkvz6ykYjJ/w3BN2ts69ja0R9nSSPpGzH yMkQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20260707; t=1792413709; x=1793018509; h=content-type:cc:to:from:subject:message-id:mime-version:date :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to :content-type; bh=xG7Y20y+nUaq6lJVPnnQYX7VFSFS/lNQS0CTHINS8qU=; b=vSUoMYChmbFeRkT8ifrDKmsNkX7bD8C+vskbTm4kS2a2HOXKjsY9hGRAj4zg+FK3md LvQh7/NF61atiDgNB9IqpGeOxRNmmqUtDttKE8u79xMx5Q0ycq/kuHN31i0lZ7v7Ppgu QLD9rmQDySgEkxfGbYmg2sjW/ewOQLh4fMuhaxIfDX4Eku2hFcM2CEjwCCJ1hUigVN/u Ym7Zm1nY1D9tNjApULO9zMORuzYgfIUZt6Kmd0EAW52UzY07wEpbVH9s+1jGQYo7r8lp 749Ss896j5WXEXqmRlxbPDnWJxkFArX2BpVBSn36vLzMZnU7yqrpyprYWgRNrbP/jARm 6Tew== X-Forwarded-Encrypted: i=1; AKwUvByuf4SM8lGWB7oYTbhfK0XohNS3KK+kvan2eVfdDXW7gs2zh2NAGrMaC9UEPAWAHY51+MoBdfhr4R7G0M4=@vger.kernel.org X-Gm-Message-State: AFq9FYK+o2Fgrv8ANCu6I66D03n38zJSfBoJLLZSIFufQWmUAVUpaoBx eSvUgRe2sKzU1G3KxeczahzI16i5NgcVJ3Wf04ys9pq2MGdrw48sutrMgQV4ZvwpOuQB165IYOo xExS/6pQbBHYGhfzFjA== X-Received: from ejcaf4.prod.google.com ([2002:a17:906:9984:b0:c3f:a907:231e]) (user=michalblk job=prod-delivery.src-stubby-dispatcher) by 2002:a17:907:c310:b0:c2e:640f:f9f0 with SMTP id a640c23a62f3a-c419a844d6cmr533170366b.5.1792413709199; Mon, 19 Oct 2026 05:41:49 -0700 (PDT) Date: Mon, 19 Oct 2026 12:41:44 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 X-Mailer: git-send-email 2.56.0.176.g3575e65682-goog Message-ID: <20261019124146.3582127-1-michalblk@google.com> Subject: [PATCH 0/2] sched_ext: Reset runnable_at when a task leaves its scheduler From: Michal Blaszczyk To: Tejun Heo , David Vernet , Andrea Righi , Changwoo Min Cc: sched-ext@lists.linux.dev, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, Kuba Piecuch , Michal Blaszczyk Content-Type: text/plain; charset="UTF-8" A task that is switched out of SCHED_EXT while it's waiting keeps its p->scx.runnable_at. When it comes back, the watchdog counts the time it spent in another class as waiting, and can abort the scheduler for a stall that didn't happen. Patch 1 restarts the wait when a task leaves its scheduler. Patch 2 adds a selftest that reproduces the spurious stall. The bug goes back to the watchdog's introduction in v6.12. The series is based on sched_ext/for-7.4, and patch 1 also applies to for-7.3-fixes. Tested in a virtme-ng VM (16 CPUs, PROVE_LOCKING): class_switch_stall fails every time without patch 1 and passes with it. The other sched_ext selftests give the same results with and without the series (enq_blocked fails in this VM either way). I used an AI coding assistant to find the bug and to write and test both patches. Michal Blaszczyk (2): sched_ext: Reset runnable_at when a task leaves its scheduler selftests/sched_ext: Add class_switch_stall test kernel/sched/ext/ext.c | 14 +- tools/testing/selftests/sched_ext/Makefile | 1 + .../sched_ext/class_switch_stall.bpf.c | 121 +++++++++ .../selftests/sched_ext/class_switch_stall.c | 240 ++++++++++++++++++ 4 files changed, 375 insertions(+), 1 deletion(-) create mode 100644 tools/testing/selftests/sched_ext/class_switch_stall.bpf.c create mode 100644 tools/testing/selftests/sched_ext/class_switch_stall.c base-commit: 1f5dde349ac2fca1f119ae8de3f0d2f1c2a8d338 -- 2.56.0.176.g3575e65682-goog