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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-Stat-Signature: euyh837889h4kp67kwo7y46ersn6w8xc X-Rspam-User: X-Rspamd-Queue-Id: 0247240011 X-Rspamd-Server: rspam01 X-HE-Tag: 1762376887-608877 X-HE-Meta: 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 cSNzXTWQ tLJVZHClwGaVi9RquOiWkDVwApxCCMOIQqQ3MrTXusGgK2YvnvqIcd9AvSwpLlqo1BPlwdlKkCQbuIGts2XJJy3o1Syl9SmFWYvxBOumz8UdlpWN8139P2LbdJDQt2mWCmod4F/KTF2Sw2t234eZuN36J3jsO3fJDW8hK6jN/Lsz1glrhwe6EpgGAO1R/kNphRqWy7okTxI+ls1PxrA4s3ek49qWN1i/R0pep6A8oD0+/Zyg= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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