From mboxrd@z Thu Jan 1 00:00:00 1970 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 AF0062F7AB1 for ; Wed, 17 Sep 2025 17:02:04 +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=1758128524; cv=none; b=dITRsb3xeNRU1m9US4wv1bqFrfJEMk+73dCR26uTGFOHNHps5XcDqy+eBHRbF4S5G0DyrEDtwxIKB7JVA5hQ2EXjM6SOmREcI+jEBspRUq/eMfKUH6S54byvrxhwd+MOUKfmIJ8nhg7efzECUNfCnV08YehkLGUlNW773N/1pTI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758128524; c=relaxed/simple; bh=KiS/9oO2llifD17emDEwPJHpkWwnjkTWX1OPZ0anfqE=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=EbElyzQIl2Tfwp4XCEQUWu7phVlQWuO/EHK6oI0ZzxKb/kKX6Fe3eX6x0rrdLzdcS/SeUwy3k1lMZn7CForwUtROepOFVs3BecXNvgByn4KWR8UA+v0uJH5W1vTjxdhVu3UA0FdMfNiWnKP7yDKV5ThAiJ/2DhyXRTbl4vdfiqg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=s4FFPYB+; 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="s4FFPYB+" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D7DA1C4CEE7; Wed, 17 Sep 2025 17:02:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1758128524; bh=KiS/9oO2llifD17emDEwPJHpkWwnjkTWX1OPZ0anfqE=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=s4FFPYB+zECbZXPvxHQidHtOcTsR+dZOmIhqoent65+yTz1nSPXJlC9zDEGmbCpfd uYdv3DnTGAXSJbzaf7iw8f17uHHu14EzgIskvhtL4jt/l5hYlBqdOoospl0xHEjHjg k+raAH8yPPm+D6tVb5XZFtaKPhyZyWYO40UszteC6dlbQq/f0UwHhqSgpWzGdbHo+q HV0nAPH7u3739an4X389PMQ2UtOWmeI2iyRWavNyOteLgG2c1eCXS1Vp/VsQ7dI/Hu oJrnnBYD8ykLYrGcQmuJcnJ3udmLGdIDvkjFgeqCcg6sJoNfR2yLvwikDbYfwVvfvi XY3s0GsKA+rvQ== Date: Wed, 17 Sep 2025 19:02:00 +0200 From: Frederic Weisbecker To: Gabriele Monaco Cc: linux-kernel@vger.kernel.org, Anna-Maria Behnsen , Thomas Gleixner , Waiman Long Subject: Re: [PATCH v13 0/9] timers: Exclude isolated cpus from timer migration Message-ID: References: <20250917161958.178925-1-gmonaco@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20250917161958.178925-1-gmonaco@redhat.com> Le Wed, Sep 17, 2025 at 06:19:49PM +0200, 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 5 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 6 and 7 adapt isolation and cpuset 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. > > Patch 9 extends the unavailable status to domain isolated CPUs, which > is the main contribution of the series. > > 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 [2]: > * 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 [3]: > * 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 [4]: > * 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/20250530142031.215594-1-gmonaco@redhat.com > [3] - https://lore.kernel.org/lkml/20250506091534.42117-7-gmonaco@redhat.com > [4] - https://lore.kernel.org/lkml/20250410065446.57304-2-gmonaco@redhat.com > > Frederic Weisbecker (1): > timers/migration: Postpone online/offline callbacks registration to > late initcall > > Gabriele Monaco (7): > 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_exclusion_cpumasks() > sched/isolation: Force housekeeping if isolcpus and nohz_full don't > leave any > cgroup/cpuset: Fail if isolated and nohz_full don't leave any > housekeeping > 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 | 78 +++++++++- > kernel/sched/isolation.c | 23 +++ > kernel/time/timer_migration.c | 205 ++++++++++++++++++++----- > kernel/time/timer_migration.h | 2 +- > 7 files changed, 278 insertions(+), 45 deletions(-) > > > base-commit: 320475fbd590dc94a0a3d9173f81e0797ee1a232 So what is the best way to route this? Timers or cgroups tree? Thanks. > -- > 2.51.0 > -- Frederic Weisbecker SUSE Labs