From nobody Thu Oct 2 15:19:17 2025 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (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 AC7B3341661 for ; Mon, 15 Sep 2025 15:00:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757948437; cv=none; b=LVXad5YmV0gXZ0Z6LbTm/13WuPg0g0VTyGdCWWTPu2YYFkICdvpSxherjzH6mwyXo+WtDJqM7/TRf87q13CbJIQ2unDxu96BoUpFCeku8yUm0wBzRBqYCSw/3u/ZyMIxbmANAOmJ4eD4pUwMV3fDEzsa60m8hONLS7cOikT+/EU= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757948437; c=relaxed/simple; bh=Gje1a6MuxbQRqJJjVBWFqMGNjvc5qcBa8yIEEkD8wWQ=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=tP21c3AmiLJkAnt5QI4WjExsRKq6HXM7fbbH7Vhx/IVNwadKLgg26Pn4yXN4WEHa6JdjQRQNDuwg+VXOBWL1Pi4WMcLgh+n9NlhwRDRRNkGz/yzACrzN7mRJwU8s64yQz0zo4iGZQeaSxiFMnkn4Uo/kIbesuAfkumlpNBqkBWI= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=f9SQ7rCz; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="f9SQ7rCz" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1757948434; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=SLP0rl8cKBQmenYpN/sNhONBZ/EaXWDugv9JYiXlVuM=; b=f9SQ7rCzpEMQECoAdGribDJWYfHXC7ybWgIif7rW3Gz4dC6s0+uriO7AvvLHDYWHImdnW4 pNipDuaEtYEGHT9bAwjrodwUnCPqnvFTxq3poH32ZRdHWLr+RA/DFpLYH/oK/AI89Zx/q7 H0PmZNo79H2W06mw2615gxYW1DR0KUc= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-687-4BA_leHXP1ml6STXCRd0wQ-1; Mon, 15 Sep 2025 11:00:30 -0400 X-MC-Unique: 4BA_leHXP1ml6STXCRd0wQ-1 X-Mimecast-MFC-AGG-ID: 4BA_leHXP1ml6STXCRd0wQ_1757948428 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 8C8A418002C4; Mon, 15 Sep 2025 15:00:28 +0000 (UTC) Received: from gmonaco-thinkpadt14gen3.rmtit.com (unknown [10.44.32.63]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 264F21800446; Mon, 15 Sep 2025 15:00:23 +0000 (UTC) From: Gabriele Monaco To: linux-kernel@vger.kernel.org, Anna-Maria Behnsen , Thomas Gleixner , Waiman Long , Tejun Heo , Johannes Weiner , =?UTF-8?q?Michal=20Koutn=C3=BD?= , cgroups@vger.kernel.org Cc: Gabriele Monaco , "John B. Wyatt IV" , "John B. Wyatt IV" Subject: [PATCH v12 9/9] timers: Exclude isolated cpus from timer migration Date: Mon, 15 Sep 2025 16:59:30 +0200 Message-ID: <20250915145920.140180-20-gmonaco@redhat.com> In-Reply-To: <20250915145920.140180-11-gmonaco@redhat.com> References: <20250915145920.140180-11-gmonaco@redhat.com> 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 X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 Content-Type: text/plain; charset="utf-8" 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) Tested-by: John B. Wyatt IV Tested-by: John B. Wyatt IV Signed-off-by: Gabriele Monaco --- include/linux/timer.h | 9 +++ kernel/cgroup/cpuset.c | 3 + kernel/time/timer_migration.c | 108 +++++++++++++++++++++++++++++++++- 3 files changed, 119 insertions(+), 1 deletion(-) diff --git a/include/linux/timer.h b/include/linux/timer.h index 0414d9e6b4fc..62e1cea71125 100644 --- a/include/linux/timer.h +++ b/include/linux/timer.h @@ -188,4 +188,13 @@ int timers_dead_cpu(unsigned int cpu); #define timers_dead_cpu NULL #endif =20 +#if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON) +extern int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask); +#else +static inline int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_c= pumask) +{ + return 0; +} +#endif + #endif diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c index 3cedc3580373..6e9d86fab27e 100644 --- a/kernel/cgroup/cpuset.c +++ b/kernel/cgroup/cpuset.c @@ -1399,6 +1399,9 @@ static void update_exclusion_cpumasks(bool isolcpus_u= pdated) =20 ret =3D workqueue_unbound_exclude_cpumask(isolated_cpus); WARN_ON_ONCE(ret < 0); + + ret =3D tmigr_isolated_exclude_cpumask(isolated_cpus); + WARN_ON_ONCE(ret < 0); } =20 /** diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c index 0a3a26e766d0..08e29fc01479 100644 --- a/kernel/time/timer_migration.c +++ b/kernel/time/timer_migration.c @@ -10,6 +10,7 @@ #include #include #include +#include =20 #include "timer_migration.h" #include "tick-internal.h" @@ -436,6 +437,29 @@ static inline bool tmigr_is_not_available(struct tmigr= _cpu *tmc) return !(tmc->tmgroup && tmc->available); } =20 +/* + * Returns true if @cpu should be excluded from the hierarchy as isolated. + * Domain isolated CPUs don't participate in timer migration, nohz_full CP= Us + * are still part of the hierarchy but become idle (from a tick and timer + * migration perspective) when they stop their tick. This lets the timekee= ping + * CPU handle their global timers. Marking also isolated CPUs as idle woul= d be + * too costly, hence they are completely excluded from the hierarchy. + * This check is necessary, for instance, to prevent offline isolated CPUs= from + * being incorrectly marked as available once getting back online. + * + * Additionally, the tick CPU can be isolated at boot, however + * we cannot mark it as unavailable to avoid having no global migrator + * for the nohz_full CPUs. This check is only necessary at boot time. + */ +static inline bool tmigr_is_isolated(int cpu) +{ + if (!tick_nohz_cpu_hotpluggable(cpu)) + return false; + return (!housekeeping_cpu(cpu, HK_TYPE_DOMAIN) || + cpuset_cpu_is_isolated(cpu)) && + housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE); +} + /* * Returns true, when @childmask corresponds to the group migrator or when= the * group is not active - so no migrator is set. @@ -1451,6 +1475,8 @@ static int tmigr_clear_cpu_available(unsigned int cpu) =20 cpumask_clear_cpu(cpu, tmigr_available_cpumask); scoped_guard(raw_spinlock_irq, &tmc->lock) { + if (!tmc->available) + return 0; tmc->available =3D false; WRITE_ONCE(tmc->wakeup, KTIME_MAX); =20 @@ -1478,8 +1504,12 @@ static int tmigr_set_cpu_available(unsigned int cpu) if (WARN_ON_ONCE(!tmc->tmgroup)) return -EINVAL; =20 + if (tmigr_is_isolated(cpu)) + return 0; cpumask_set_cpu(cpu, tmigr_available_cpumask); scoped_guard(raw_spinlock_irq, &tmc->lock) { + if (tmc->available) + return 0; trace_tmigr_cpu_available(tmc); tmc->idle =3D timer_base_is_idle(); if (!tmc->idle) @@ -1489,6 +1519,81 @@ static int tmigr_set_cpu_available(unsigned int cpu) return 0; } =20 +static void tmigr_cpu_isolate(struct work_struct *ignored) +{ + tmigr_clear_cpu_available(smp_processor_id()); +} + +static void tmigr_cpu_unisolate(struct work_struct *ignored) +{ + tmigr_set_cpu_available(smp_processor_id()); +} + +/** + * tmigr_isolated_exclude_cpumask - Exclude given CPUs from hierarchy + * @exclude_cpumask: the cpumask to be excluded from timer migration hiera= rchy + * + * This function can be called from cpuset code to provide the new set of + * isolated CPUs that should be excluded from the hierarchy. + * Online CPUs not present in exclude_cpumask but already excluded are bro= ught + * back to the hierarchy. + * Functions to isolate/unisolate need to be called locally and can sleep. + */ +int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask) +{ + struct work_struct __percpu *works __free(free_percpu) =3D + alloc_percpu(struct work_struct); + cpumask_var_t cpumask_unisol __free(free_cpumask_var) =3D CPUMASK_NULL; + cpumask_var_t cpumask_isol __free(free_cpumask_var) =3D CPUMASK_NULL; + int cpu; + + lockdep_assert_cpus_held(); + + if (!alloc_cpumask_var(&cpumask_isol, GFP_KERNEL)) + return -ENOMEM; + if (!alloc_cpumask_var(&cpumask_unisol, GFP_KERNEL)) + return -ENOMEM; + if (!works) + return -ENOMEM; + + cpumask_andnot(cpumask_unisol, cpu_online_mask, exclude_cpumask); + cpumask_andnot(cpumask_unisol, cpumask_unisol, tmigr_available_cpumask); + /* Set up the mask earlier to avoid races with the migrator CPU */ + cpumask_or(tmigr_available_cpumask, tmigr_available_cpumask, cpumask_unis= ol); + for_each_cpu(cpu, cpumask_unisol) { + struct work_struct *work =3D per_cpu_ptr(works, cpu); + + INIT_WORK(work, tmigr_cpu_unisolate); + schedule_work_on(cpu, work); + } + + cpumask_and(cpumask_isol, exclude_cpumask, tmigr_available_cpumask); + cpumask_and(cpumask_isol, cpumask_isol, housekeeping_cpumask(HK_TYPE_KERN= EL_NOISE)); + /* + * Handle this here and not in the cpuset code because exclude_cpumask + * might include also the tick CPU if included in isolcpus. + */ + for_each_cpu(cpu, cpumask_isol) { + if (!tick_nohz_cpu_hotpluggable(cpu)) { + cpumask_clear_cpu(cpu, cpumask_isol); + break; + } + } + /* Set up the mask earlier to avoid races with the migrator CPU */ + cpumask_andnot(tmigr_available_cpumask, tmigr_available_cpumask, cpumask_= isol); + for_each_cpu(cpu, cpumask_isol) { + struct work_struct *work =3D per_cpu_ptr(works, cpu); + + INIT_WORK(work, tmigr_cpu_isolate); + schedule_work_on(cpu, work); + } + + for_each_cpu_or(cpu, cpumask_isol, cpumask_unisol) + flush_work(per_cpu_ptr(works, cpu)); + + return 0; +} + /* * NOHZ can only be enabled after clocksource_done_booting(). Don't * bother trashing the cache in the tree before. @@ -1496,7 +1601,8 @@ static int tmigr_set_cpu_available(unsigned int cpu) static int __init tmigr_late_init(void) { return cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE, "tmigr:online", - tmigr_set_cpu_available, tmigr_clear_cpu_available); + tmigr_set_cpu_available, + tmigr_clear_cpu_available); } =20 static void tmigr_init_group(struct tmigr_group *group, unsigned int lvl, --=20 2.51.0