Message-ID: <aRXZTljABwNiZDsF@localhost.localdomain>
Date: Thu, 13 Nov 2025 14:12:46 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: linux-kernel@...r.kernel.org,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v15 0/7] timers: Exclude isolated cpus from timer
 migration

Le Thu, Nov 13, 2025 at 09:33:17AM +0100, Gabriele Monaco a écrit :
> The timer migration mechanism allows active CPUs to pull timers from
> idle ones to improve the overall idle time. This is however undesired
> when CPU intensive workloads run on isolated cores, as the algorithm
> would move the timers from housekeeping to isolated cores, negatively
> affecting the isolation.
> 
> Exclude isolated cores from the timer migration algorithm, extend the
> concept of unavailable cores, currently used for offline ones, to
> isolated ones:
> * A core is unavailable if isolated or offline;
> * A core is available if non isolated and online;
> 
> A core is considered unavailable as isolated if it belongs to:
> * the isolcpus (domain) list
> * an isolated cpuset
> Except if it is:
> * in the nohz_full list (already idle for the hierarchy)
> * the nohz timekeeper core (must be available to handle global timers)
> 
> CPUs are added to the hierarchy during late boot, excluding isolated
> ones, the hierarchy is also adapted when the cpuset isolation changes.
> 
> Due to how the timer migration algorithm works, any CPU part of the
> hierarchy can have their global timers pulled by remote CPUs and have to
> pull remote timers, only skipping pulling remote timers would break the
> logic.
> For this reason, prevent isolated CPUs from pulling remote global
> timers, but also the other way around: any global timer started on an
> isolated CPU will run there. This does not break the concept of
> isolation (global timers don't come from outside the CPU) and, if
> considered inappropriate, can usually be mitigated with other isolation
> techniques (e.g. IRQ pinning).
> 
> This effect was noticed on a 128 cores machine running oslat on the
> isolated cores (1-31,33-63,65-95,97-127). The tool monopolises CPUs,
> and the CPU with lowest count in a timer migration hierarchy (here 1
> and 65) appears as always active and continuously pulls global timers,
> from the housekeeping CPUs. This ends up moving driver work (e.g.
> delayed work) to isolated CPUs and causes latency spikes:
> 
> before the change:
> 
>  # oslat -c 1-31,33-63,65-95,97-127 -D 62s
>  ...
>   Maximum:     1203 10 3 4 ... 5 (us)
> 
> after the change:
> 
>  # oslat -c 1-31,33-63,65-95,97-127 -D 62s
>  ...
>   Maximum:      10 4 3 4 3 ... 5 (us)
> 
> The same behaviour was observed on a machine with as few as 20 cores /
> 40 threads with isocpus set to: 1-9,11-39 with rtla-osnoise-top.
> 
> The first 4 patches are preparatory work to change the concept of
> online/offline to available/unavailable, keep track of those in a
> separate cpumask cleanup the setting/clearing functions and change a
> function name in cpuset code.
> 
> Patch 5 adapt isolation to prevent domain isolated and nohz_full from
> covering all CPUs not leaving any housekeeping one. This can lead to
> problems with the changes introduced in this series because no CPU would
> remain to handle global timers.
> (The corresponding change for cpuset was removed from this version of
> the series and is present in [2]).
> 
> Patch 7 extends the unavailable status to domain isolated CPUs, which
> is the main contribution of the series.
> 
> Changes since v14:
> * Rebase on tip/timers/core, rename one more ->online field
> * Mark the static key as static
> * Share code between tmigr_init_isolation and tmigr_isolated_exclude_cpumask
> 
> Changes since v13:
> * Remove tmigr late initialisation and restore late isolation (as in v8 [1])
> * Use workqueues in initialisation just like tmigr_available_cpumask()
> * Use static key for tmigr_exclude_isolated
> * Remove cpuset patch checking for HK conflict (included in [2])
> * Rename cpuset helper to update_isolation_cpumasks as in [2]
> 
> Changes since v12:
> * Pick and adapt patch by Yury Norov to initialise cpumasks
> * Reorganise accesses to tmigr_available_cpumask to avoid races
> 
> Changes since v11:
> * Rename isolcpus_nohz_conflict() to isolated_cpus_can_update()
> * Move tick_nohz_cpu_hotpluggable() check to tmigr_is_isolated()
> * Use workqueues in tmigr_isolated_exclude_cpumask() to avoid sleeping
>   while atomic
> * Add cpumask initialiser to safely use cpumask cleanup helpers
> 
> Changes since v10:
> * Simplify housekeeping conflict condition
> * Reword commit (Frederic Weisbecker)
> 
> Changes since v9:
> * Fix total housekeeping enforcement to focus only on nohz and domain
> * Avoid out of bound access in the housekeeping array if no flag is set
> * Consider isolated_cpus while checking for nohz conflicts in cpuset
> * Improve comment about why nohz CPUs are not excluded by tmigr
> 
> Changes since v8 [1]:
> * Postpone hotplug registration to late initcall (Frederic Weisbecker)
> * Move main activation logic in _tmigr_set_cpu_available() and call it
>   after checking for isolation on hotplug and cpusets changes
> * Call _tmigr_set_cpu_available directly to force enable tick CPU if
>   required (this saves checking for that on every hotplug change).
> 
> Changes since v7:
> * Move tmigr_available_cpumask out of tmc lock and specify conditions.
> * Initialise tmigr isolation despite the state of isolcpus.
> * Move tick CPU check to condition to run SMP call.
> * Fix descriptions.
> 
> Changes since v6 [3]:
> * Prevent isolation checks from running during early boot
> * Prevent double (de)activation while setting cpus (un)available
> * Use synchronous smp calls from the isolation path
> * General cleanup
> 
> Changes since v5:
> * Remove fallback if no housekeeping is left by isolcpus and nohz_full
> * Adjust condition not to activate CPUs in the migration hierarchy
> * Always force the nohz tick CPU active in the hierarchy
> 
> Changes since v4 [4]:
> * use on_each_cpu_mask() with changes on isolated CPUs to avoid races
> * keep nohz_full CPUs included in the timer migration hierarchy
> * prevent domain isolated and nohz_full to cover all CPUs
> 
> Changes since v3:
> * add parameter to function documentation
> * split into multiple straightforward patches
> 
> Changes since v2:
> * improve comments about handling CPUs isolated at boot
> * minor cleanup
> 
> Changes since v1 [5]:
> * split into smaller patches
> * use available mask instead of unavailable
> * simplification and cleanup
> 
> [1] - https://lore.kernel.org/lkml/20250714133050.193108-9-gmonaco@redhat.com
> [2] - https://lore.kernel.org/lkml/20251104013037.296013-1-longman@redhat.com
> [3] - https://lore.kernel.org/lkml/20250530142031.215594-1-gmonaco@redhat.com
> [4] - https://lore.kernel.org/lkml/20250506091534.42117-7-gmonaco@redhat.com
> [5] - https://lore.kernel.org/lkml/20250410065446.57304-2-gmonaco@redhat.com
> 
> Gabriele Monaco (6):
>   timers: Rename tmigr 'online' bit to 'available'
>   timers: Add the available mask in timer migration
>   timers: Use scoped_guard when setting/clearing the tmigr available
>     flag
>   cgroup/cpuset: Rename update_unbound_workqueue_cpumask() to
>     update_isolation_cpumasks()
>   sched/isolation: Force housekeeping if isolcpus and nohz_full don't
>     leave any
>   timers: Exclude isolated cpus from timer migration
> 
> Yury Norov (1):
>   cpumask: Add initialiser to use cleanup helpers
> 
>  include/linux/cpumask.h                |   2 +
>  include/linux/timer.h                  |   9 ++
>  include/trace/events/timer_migration.h |   4 +-
>  kernel/cgroup/cpuset.c                 |  15 +-
>  kernel/sched/isolation.c               |  23 +++
>  kernel/time/timer_migration.c          | 213 +++++++++++++++++++++----
>  kernel/time/timer_migration.h          |   2 +-
>  7 files changed, 232 insertions(+), 36 deletions(-)
> 
> 
> base-commit: ba14500e4bfcab5e841fbf8d7fcbbc80e98d6b9e

Looks good to me now, thanks!

-- 
Frederic Weisbecker
SUSE Labs

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