From nobody Sat Feb 7 21:14:39 2026 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 461AE31195C; Sun, 25 Jan 2026 22:50:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769381421; cv=none; b=P6LzzLpJaxISPc8th48b61ZaV61CNJ0ZBexvVIyfWEFX4vaz1How2Q8BG+LOpddzrixnewXk6WSEGah4HX6CmWxCc1XrU+zdSOmhDZhp1wj4t/hvHFsi3w0cXZKMlWKh+a4GIUkg3agG1gx5DP1+XG7wqhSx+xboXRqU8iuvql8= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769381421; c=relaxed/simple; bh=soOg+pSguRYShDvlBT6z+CExBTh9mpEwKqBDpU+MvVA=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=r211aD/MjNFJP/I/vC720IJbw7NyF9nmniDbdW+3xKNC7OzssMzM5lqxhiOv4ge/1rfVjO641tmfgMWwFBRj7I4QdGVaIFfup2+c7DLC4Yqj2eUBMjMkQwjPlGdw9OHPoBTdV7ojuqwpfvr/3xlkpvPcrvuzzLas5IZBCIm8duM= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=O9eNU6FB; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="O9eNU6FB" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 63F9BC16AAE; Sun, 25 Jan 2026 22:50:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1769381420; bh=soOg+pSguRYShDvlBT6z+CExBTh9mpEwKqBDpU+MvVA=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=O9eNU6FBgsG9hyVi/+uWzwzBcb3jcShb7uOmQsjIIjUrQdwd6h2E2hlOYXFcXB/ql qdKPPIOMoMFnsXK4JR327RdmvVp12tzAKc3mdCuECy36dJ+ULwoOd7XxIR96MFfnRS Tf/BhWFSthB4dknZzQT+VDMKWaiGk1piYc4/1SvoEyWQPVWOMbGzCOX50wyM0HhBgo zQY6jjB6xI7BeQI9xaby26nk/VdbQlcrksM8alFh3VGgf4VPNgHrdrPRTG1yHtNs6B LJOghmDwx1hiHEaoiXnunqOam5DBew91Q1zeucjxGqoQn4ndm/rwNY/R4fMGcCaLal ZHqk/ey3wxMIw== From: Frederic Weisbecker To: LKML Cc: Frederic Weisbecker , =?UTF-8?q?Michal=20Koutn=C3=BD?= , Andrew Morton , Bjorn Helgaas , Catalin Marinas , Chen Ridong , 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 33/33] doc: Add housekeeping documentation Date: Sun, 25 Jan 2026 23:45:40 +0100 Message-ID: <20260125224541.50226-34-frederic@kernel.org> X-Mailer: git-send-email 2.51.1 In-Reply-To: <20260125224541.50226-1-frederic@kernel.org> References: <20260125224541.50226-1-frederic@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Signed-off-by: Frederic Weisbecker Acked-by: Waiman Long --- Documentation/core-api/housekeeping.rst | 111 ++++++++++++++++++++++++ Documentation/core-api/index.rst | 1 + 2 files changed, 112 insertions(+) create mode 100644 Documentation/core-api/housekeeping.rst diff --git a/Documentation/core-api/housekeeping.rst b/Documentation/core-a= pi/housekeeping.rst new file mode 100644 index 000000000000..e5417302774c --- /dev/null +++ b/Documentation/core-api/housekeeping.rst @@ -0,0 +1,111 @@ +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +Housekeeping +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + + +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=3Ddomain`` 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=3D`` or ``isolcpus=3Dnohz`` 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=3Dmanaged_irq``. + +4. HK_TYPE_DOMAIN_BOOT matches the work moved away by scheduler domain + isolation performed through ``isolcpus=3Ddomain`` only. It is similar + to HK_TYPE_DOMAIN except it ignores the isolation performed by + cpusets. + + +Housekeeping cpumasks +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D + +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=3D``, 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=3Ddomain,7 the following will return a mask with all possible +CPUs except 7:: + + housekeeping_cpumask(HK_TYPE_DOMAIN) + +Similarly with nohz_full=3D5,6 the following will return a mask with all +possible CPUs except 5,6:: + + housekeeping_cpumask(HK_TYPE_KERNEL_NOISE) + + +Synchronization against cpusets +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D + +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 upda= te +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 with= in +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 =3D housekeeping_any_cpu(HK_TYPE_DOMAIN); + queue_work_on(cpu, example_workqueue, work); + rcu_read_unlock(); diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/inde= x.rst index 5eb0fbbbc323..79fe7735692e 100644 --- a/Documentation/core-api/index.rst +++ b/Documentation/core-api/index.rst @@ -25,6 +25,7 @@ it. symbol-namespaces asm-annotations real-time/index + housekeeping.rst =20 Data structures and low-level utilities =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D --=20 2.51.1