From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id C2B6BCCFA0D for ; Wed, 5 Nov 2025 21:08:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: MIME-Version:References:In-Reply-To:Message-ID:Date:Subject:Cc:To:From: Reply-To:Content-Type:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=N1PNdgknOyyk2mzZmr1xYTl18CtGDJfdBKGGVXljgnw=; b=wxrFi8dHJtJBgUpaOfNRCH8kUR CLuO8TbWqhk5nR5SS+wggPIV272wmwcr4GVkgkyYXy7r5/HzFQffGwPVG90AZVTIPMcJF1/lLsFnW V5VfP/bYOG5woN2uhSMkT9JgJI6bkEbtbyawACNYkuXCaLPVpnrLJDj64R27sueBP9U4X+usBOREr 5fECnZciYAsPTJBTwVGOWXty4JsH5JKVKeXLWVmd2LnfT/+Bwl3u2InJbPY9TWvLB2f5zYqhDY9qh mD9xScZV5Ve677d8YShUJAfAcgBv/oGoOVLn7O+404vNturUX21jzd0bnotzLo08QGBFq+r5w1l9B OdYPMU4g==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vGkjh-0000000ESYZ-1Tol; Wed, 05 Nov 2025 21:08:09 +0000 Received: from tor.source.kernel.org ([2600:3c04:e001:324:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1vGkjg-0000000ESXV-0NjF for linux-arm-kernel@lists.infradead.org; Wed, 05 Nov 2025 21:08:08 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by tor.source.kernel.org (Postfix) with ESMTP id 7CF136021F; Wed, 5 Nov 2025 21:08:07 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4D4C2C116C6; Wed, 5 Nov 2025 21:07:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1762376887; bh=Jrc2SP9zuO/q8lVKXeMSZQ2TjjYp5fXU9+UkI2oxxh8=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=MvMmy9z/Tgk53O6EbE+zG2oKCMSZwHnfkHuPoSeCh479KVbDuJNzsb/Q95ddbdX1O YQTf2wfL/dngH/FTqEV6nyuyONeRGG6HmXIoUpQYnvRwoBce1dGez5Rnclx/R54AJG jF46uVQGSBmc+QMKekLAz3yISaEfO/lEUBh/VXH4j9x5/+pRiLN7gYF1+fWkOKQ36F j1wBHxPHnmeF1FuhdLWetNBKgWb0hoEHcy6zOXnoMX7DeUEMTwVgqvZecZg7n+SX65 jdNVvmiqi+lM6qGacp0TkqxYpP3U9TZXPPfxP2T1eiIo+XV5uRXCcGxIxzrr4Zr+Kh OUYcrvQZvJ2Zg== From: Frederic Weisbecker To: LKML Cc: Frederic Weisbecker , =?UTF-8?q?Michal=20Koutn=C3=BD?= , Andrew Morton , Bjorn Helgaas , Catalin Marinas , Danilo Krummrich , "David S . Miller" , Eric Dumazet , Gabriele Monaco , Greg Kroah-Hartman , Ingo Molnar , Jakub Kicinski , Jens Axboe , Johannes Weiner , Lai Jiangshan , Marco Crivellari , Michal Hocko , Muchun Song , Paolo Abeni , Peter Zijlstra , Phil Auld , "Rafael J . Wysocki" , Roman Gushchin , Shakeel Butt , Simon Horman , Tejun Heo , Thomas Gleixner , Vlastimil Babka , Waiman Long , Will Deacon , cgroups@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-block@vger.kernel.org, linux-mm@kvack.org, linux-pci@vger.kernel.org, netdev@vger.kernel.org Subject: [PATCH 31/31] doc: Add housekeeping documentation Date: Wed, 5 Nov 2025 22:03:47 +0100 Message-ID: <20251105210348.35256-32-frederic@kernel.org> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20251105210348.35256-1-frederic@kernel.org> References: <20251105210348.35256-1-frederic@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Signed-off-by: Frederic Weisbecker --- Documentation/cpu_isolation/housekeeping.rst | 111 +++++++++++++++++++ 1 file changed, 111 insertions(+) create mode 100644 Documentation/cpu_isolation/housekeeping.rst diff --git a/Documentation/cpu_isolation/housekeeping.rst b/Documentation/cpu_isolation/housekeeping.rst new file mode 100644 index 000000000000..e5417302774c --- /dev/null +++ b/Documentation/cpu_isolation/housekeeping.rst @@ -0,0 +1,111 @@ +====================================== +Housekeeping +====================================== + + +CPU Isolation moves away kernel work that may otherwise run on any CPU. +The purpose of its related features is to reduce the OS jitter that some +extreme workloads can't stand, such as in some DPDK usecases. + +The kernel work moved away by CPU isolation is commonly described as +"housekeeping" because it includes ground work that performs cleanups, +statistics maintainance and actions relying on them, memory release, +various deferrals etc... + +Sometimes housekeeping is just some unbound work (unbound workqueues, +unbound timers, ...) that gets easily assigned to non-isolated CPUs. +But sometimes housekeeping is tied to a specific CPU and requires +elaborated tricks to be offloaded to non-isolated CPUs (RCU_NOCB, remote +scheduler tick, etc...). + +Thus, a housekeeping CPU can be considered as the reverse of an isolated +CPU. It is simply a CPU that can execute housekeeping work. There must +always be at least one online housekeeping CPU at any time. The CPUs that +are not isolated are automatically assigned as housekeeping. + +Housekeeping is currently divided in four features described +by the ``enum hk_type type``: + +1. HK_TYPE_DOMAIN matches the work moved away by scheduler domain + isolation performed through ``isolcpus=domain`` boot parameter or + isolated cpuset partitions in cgroup v2. This includes scheduler + load balancing, unbound workqueues and timers. + +2. HK_TYPE_KERNEL_NOISE matches the work moved away by tick isolation + performed through ``nohz_full=`` or ``isolcpus=nohz`` boot + parameters. This includes remote scheduler tick, vmstat and lockup + watchdog. + +3. HK_TYPE_MANAGED_IRQ matches the IRQ handlers moved away by managed + IRQ isolation performed through ``isolcpus=managed_irq``. + +4. HK_TYPE_DOMAIN_BOOT matches the work moved away by scheduler domain + isolation performed through ``isolcpus=domain`` only. It is similar + to HK_TYPE_DOMAIN except it ignores the isolation performed by + cpusets. + + +Housekeeping cpumasks +================================= + +Housekeeping cpumasks include the CPUs that can execute the work moved +away by the matching isolation feature. These cpumasks are returned by +the following function:: + + const struct cpumask *housekeeping_cpumask(enum hk_type type) + +By default, if neither ``nohz_full=``, nor ``isolcpus``, nor cpuset's +isolated partitions are used, which covers most usecases, this function +returns the cpu_possible_mask. + +Otherwise the function returns the cpumask complement of the isolation +feature. For example: + +With isolcpus=domain,7 the following will return a mask with all possible +CPUs except 7:: + + housekeeping_cpumask(HK_TYPE_DOMAIN) + +Similarly with nohz_full=5,6 the following will return a mask with all +possible CPUs except 5,6:: + + housekeeping_cpumask(HK_TYPE_KERNEL_NOISE) + + +Synchronization against cpusets +================================= + +Cpuset can modify the HK_TYPE_DOMAIN housekeeping cpumask while creating, +modifying or deleting an isolated partition. + +The users of HK_TYPE_DOMAIN cpumask must then make sure to synchronize +properly against cpuset in order to make sure that: + +1. The cpumask snapshot stays coherent. + +2. No housekeeping work is queued on a newly made isolated CPU. + +3. Pending housekeeping work that was queued to a non isolated + CPU which just turned isolated through cpuset must be flushed + before the related created/modified isolated partition is made + available to userspace. + +This synchronization is maintained by an RCU based scheme. The cpuset update +side waits for an RCU grace period after updating the HK_TYPE_DOMAIN +cpumask and before flushing pending works. On the read side, care must be +taken to gather the housekeeping target election and the work enqueue within +the same RCU read side critical section. + +A typical layout example would look like this on the update side +(``housekeeping_update()``):: + + rcu_assign_pointer(housekeeping_cpumasks[type], trial); + synchronize_rcu(); + flush_workqueue(example_workqueue); + +And then on the read side:: + + rcu_read_lock(); + cpu = housekeeping_any_cpu(HK_TYPE_DOMAIN); + queue_work_on(cpu, example_workqueue, work); + rcu_read_unlock(); -- 2.51.0