From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id DA6463B27CD for ; Tue, 28 Apr 2026 13:04:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777381491; cv=none; b=jRzx8xzT+VtAw1E3hPrc48ezxX0Kt4vOdXMHaKJ+9JoBovggcp7Zqdt9DFk1/4nYQL9R0zhyLGnanB3ZbCRpVfkl8RsRKPEECtR+FD0pxT1Ig7LQsyEaDSveWcT+7JdFVl+sHPe8sQQyHaH5Lm5mPEC5Ee72sUJ92ytovO7m6Yk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777381491; c=relaxed/simple; bh=9gdcDRenUGFKw5PsNJIFLpW5N1xPO3UA+P8GUeN/84E=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=hC+IYwk3aK07XPzlSxK7pkfsaecW4s8B4oYxmSjk4NSIWb77DUqhHynxcSKSXT4phoOjtITF2s2A5N7c35UHkFacHGk8+ojQ+R+m+wNXJkWxF7S3Q8VhApqCoeQPr+ZlvxewgPxCwO3Wa+hZ4QAt/YSpiWz3gJTppNJmO5QP8Eg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=lwIKlhyO; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="lwIKlhyO" Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id D58402BCB; Tue, 28 Apr 2026 06:04:43 -0700 (PDT) Received: from e134344.cambridge.arm.com (e134344.arm.com [10.1.196.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id 38A223F62B; Tue, 28 Apr 2026 06:04:47 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1777381489; bh=9gdcDRenUGFKw5PsNJIFLpW5N1xPO3UA+P8GUeN/84E=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=lwIKlhyOLP5Hpdh7mWlhpt4Jbefj0a7be051gHhXo7Bxs/1AsK8Df+D/CIGLTRNEU PmmFFRprG6MyEiZj9AQfAqZ+hX8vgPQqKMnN6uwvMenM7piALUboWckbUGAQGH/nOe Nn9hGQXkkVlNYbT5L57vZA52eNfD+8Hja9W6E7uw= From: Ben Horgan To: linux-kernel@vger.kernel.org Cc: tony.luck@intel.com, reinette.chatre@intel.com, Dave.Martin@arm.com, james.morse@arm.com, babu.moger@amd.com, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, ben.horgan@arm.com, fenghuay@nvidia.com, tan.shaopeng@fujitsu.com Subject: [PATCH v5 7/7] fs/resctrl: Document tasks file behaviour for task id 0 and idle tasks Date: Tue, 28 Apr 2026 14:04:22 +0100 Message-ID: <20260428130422.2287302-8-ben.horgan@arm.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260428130422.2287302-1-ben.horgan@arm.com> References: <20260428130422.2287302-1-ben.horgan@arm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit When 0 is written to the tasks file it is interpreted as the current task in rdtgroup_move_task(). Each CPU's idle task has task_struct::pid set to 0 and, on x86, task_struct::closid to RESCTRL_RESERVED_CLOSID and task_struct::rmid to RESCTRL_RESERVED_RMID. Equivalently, on MPAM platforms, thread_info::mpam_partid_pmg is encoded with PARTID and PMG set to RESCTRL_RESERVED_CLOSID and RESCTRL_RESERVED_RMID, respectively. As there is no interface to change these from the default, the resctrl configuration for the idle tasks is fixed and they always behave equivalently to a task in the default tasks file and so take their configuration from the cpus/cpus_list files. On read of the tasks file, show_rdt_tasks() filters out any 0 PID. Hence, a task id of 0 is never shown in the tasks file and the idle tasks are not represented either. Document the user visible behaviour. Signed-off-by: Ben Horgan --- I have confirmed this experimentally on an MPAM platform. Changes since v4: Clean up the commit message (Reinette) --- Documentation/filesystems/resctrl.rst | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst index b388e9193896..e4b66af55ffb 100644 --- a/Documentation/filesystems/resctrl.rst +++ b/Documentation/filesystems/resctrl.rst @@ -575,6 +575,11 @@ All groups contain the following files: then the task must already belong to the CTRL_MON parent of this group. The task is removed from any previous MON group. + When writing to this file, a task id of 0 is interpreted as the + task id of the currently running task. On reading the file, a task + id of 0 will never be shown and there is no representation of the + idle tasks. Instead, a CPU's idle task is always considered as a + member of the group owning the CPU. "cpus": Reading this file shows a bitmask of the logical CPUs owned by -- 2.43.0