From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mgamail.intel.com (mgamail.intel.com [198.175.65.14]) (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 035AD39EF35 for ; Tue, 28 Jul 2026 20:16:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.175.65.14 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785269780; cv=none; b=rd9/iR6yZIE+z3o6dxACXSlHIN1RpAm26MYdQG+6npve9Zu1Np917BU5qsnL4bk7FdP0Qf53V8V4a5+mkMhFXVt0LU+FOmwnNJrmZ1YRu1NV9kiEhGpqgj0nTiYMgV2uuSptSho4BV0fo4aKgKbf5xtZzv2Dhgz/wdHHZ9foIPY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785269780; c=relaxed/simple; bh=hKTCJIXnLdHCW0nJd/7e85CcfQd2xj1/7pL1WHU7PWE=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=FJtotuk1slBcgCJ9rAjVeiHo+gt/T052bgDLoWmsSpn44J54ce2aPDhCq4II4jZ0U/dO+MRy2GpTBK3IPvryaJJipdivDrwvENPfyPWInSrqDcTFzvg7ZCJrC3/Tg44XEYmQJIai0j912dq9Rk8X4dx62Jic9SRB5ceqI0jvyy8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.intel.com; spf=pass smtp.mailfrom=linux.intel.com; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b=a0akMxy8; arc=none smtp.client-ip=198.175.65.14 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.intel.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.intel.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b="a0akMxy8" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1785269778; x=1816805778; h=message-id:subject:from:to:cc:date:in-reply-to: references:content-transfer-encoding:mime-version; bh=hKTCJIXnLdHCW0nJd/7e85CcfQd2xj1/7pL1WHU7PWE=; b=a0akMxy8aVuovVw8eoAUMZWjl32rPkOLMaDRoieQCUlQNn3W6uo7DM7j XjL0dT61h4ZkLJlyitKZYKUeQIMu6IPoCB5OXNLXcAy+prBcO0nk+DMO2 H6hTBd6qllZqMvD74eLIuwLVdA2+TVIRidF+eGSEkOhri/d5sNdpHoPjO m5fWgbb04WuDGCcIJDNrvkBB0V1iJeH3SHRb+wnIui3YP45g7z3yVvKE0 FqYpVtbKDFRvAEvrQNQyihxdr9yxCuLRFHSncxh7qm41vaCEZU/iHZAPn VDdnOujDiVX3EZfVDkmtIa41p4jPkzwZgw1BwkwDWL9/GLHkvRAYcDAEV w==; X-CSE-ConnectionGUID: yKCBPQD4TE6exGdqaJC+Yg== X-CSE-MsgGUID: LYtO3pujSQmQq+PfxxYv/Q== X-IronPort-AV: E=McAfee;i="6800,10657,11859"; a="89756821" X-IronPort-AV: E=Sophos;i="6.25,191,1779174000"; d="scan'208";a="89756821" Received: from fmviesa002.fm.intel.com ([10.60.135.142]) by orvoesa106.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 28 Jul 2026 13:16:17 -0700 X-CSE-ConnectionGUID: 8cEWH7YzQdiIss5T+iJJ4g== X-CSE-MsgGUID: 6tnRK7OuSc6XfmKAC3hQ6w== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.25,191,1779174000"; d="scan'208";a="283144059" Received: from unknown (HELO [10.241.243.185]) ([10.241.243.185]) by fmviesa002-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 28 Jul 2026 13:16:16 -0700 Message-ID: Subject: Re: [PATCH v8 1/2] sched/cache: Reduce the overhead of task_cache_work by only scan the visisted cpus From: Tim Chen To: Luo Gengkun , peterz@infradead.org, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, yu.c.chen@intel.com Cc: dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, kprateek.nayak@amd.com, linux-kernel@vger.kernel.org Date: Tue, 28 Jul 2026 13:16:16 -0700 In-Reply-To: <20260723040429.630176-2-luogengkun2@huawei.com> References: <20260723040429.630176-1-luogengkun2@huawei.com> <20260723040429.630176-2-luogengkun2@huawei.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.58.1 (3.58.1-1.fc43) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Thu, 2026-07-23 at 04:04 +0000, Luo Gengkun wrote: > The overhead of task_cache_work() is high, especially in multi-NUMA syste= ms. > Currently, task_cache_work() tries to find the pref_llc by scanning all C= PUs > in the system. However, most of these scans are meaningless, such as thos= e > for CPUs that have never been visited or were accessed a long time ago. >=20 > To address this problem, introduce visited_cpus to track the visited CPUs > and evict them once they have not been accessed for a duration exceeding > llc_epoch_affinity_timeout. >=20 > Now that we know exactly which CPUs to scan from visited_cpus, we can rem= ove > get_scan_cpumasks(). Thanks. The code looks good to me. Reviewed-by: Tim Chen >=20 > Signed-off-by: Luo Gengkun > --- > include/linux/mm_types.h | 6 +++ > include/linux/sched.h | 2 + > kernel/sched/fair.c | 94 ++++++++++++++++------------------------ > 3 files changed, 46 insertions(+), 56 deletions(-) >=20 > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index b18c2b2e7d2c..35559079e4d4 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -1620,6 +1620,11 @@ static inline int mm_alloc_sched_noprof(struct mm_= struct *mm) > if (!pcpu_sched) > return -ENOMEM; > =20 > + if (!zalloc_cpumask_var(&mm->sc_stat.visited_cpus, GFP_KERNEL)) { > + free_percpu(pcpu_sched); > + return -ENOMEM; > + } > + > mm_init_sched(mm, pcpu_sched); > return 0; > } > @@ -1630,6 +1635,7 @@ static inline void mm_destroy_sched(struct mm_struc= t *mm) > { > free_percpu(mm->sc_stat.pcpu_sched); > mm->sc_stat.pcpu_sched =3D NULL; > + free_cpumask_var(mm->sc_stat.visited_cpus); > } > #else /* !CONFIG_SCHED_CACHE */ > =20 > diff --git a/include/linux/sched.h b/include/linux/sched.h > index 373bcc0598d1..b461a71a65da 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -2388,6 +2388,7 @@ static __always_inline int task_mm_cid(struct task_= struct *t) > struct sched_cache_time { > u64 runtime; > unsigned long epoch; > + unsigned long epoch_last_visit; > }; > =20 > struct sched_cache_stat { > @@ -2398,6 +2399,7 @@ struct sched_cache_stat { > unsigned long next_scan; > unsigned long footprint; > int cpu; > + cpumask_var_t visited_cpus; > } ____cacheline_aligned_in_smp; > =20 > #else > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index d78467ec6ee1..10d442074f21 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -1585,6 +1585,7 @@ void mm_init_sched(struct mm_struct *mm, > pcpu_sched->runtime =3D 0; > /* a slightly stale cpu epoch is acceptible */ > pcpu_sched->epoch =3D rq->cpu_epoch; > + pcpu_sched->epoch_last_visit =3D rq->cpu_epoch; > epoch =3D rq->cpu_epoch; > } > =20 > @@ -1635,13 +1636,23 @@ static inline void __update_mm_sched(struct rq *r= q, > } > } > =20 > -static unsigned long fraction_mm_sched(struct rq *rq, > - struct sched_cache_time *pcpu_sched) > +static unsigned long fraction_mm_sched(int cpu, > + struct mm_struct *mm) > { > + struct sched_cache_time *pcpu_sched =3D > + per_cpu_ptr(mm->sc_stat.pcpu_sched, cpu); > + struct rq *rq =3D cpu_rq(cpu); > + > guard(raw_spinlock_irqsave)(&rq->cpu_epoch_lock); > =20 > __update_mm_sched(rq, pcpu_sched); > =20 > + /* Skip the rq that has not been hit for a long time */ > + if ((rq->cpu_epoch - pcpu_sched->epoch_last_visit) > llc_epoch_affinity= _timeout) { > + cpumask_clear_cpu(cpu, mm->sc_stat.visited_cpus); > + return 0; > + } > + > /* > * Runtime is a geometric series (r=3D0.5) and as such will sum to twic= e > * the accumulation period, this means the multiplcation here should > @@ -1711,6 +1722,9 @@ void account_mm_sched(struct rq *rq, struct task_st= ruct *p, s64 delta_exec) > pcpu_sched->runtime +=3D delta_exec; > rq->cpu_runtime +=3D delta_exec; > epoch =3D rq->cpu_epoch; > + pcpu_sched->epoch_last_visit =3D epoch; > + if (!cpumask_test_cpu(cpu_of(rq), mm->sc_stat.visited_cpus)) > + cpumask_set_cpu(cpu_of(rq), mm->sc_stat.visited_cpus); > } > =20 > /* > @@ -1761,51 +1775,6 @@ static void task_tick_cache(struct rq *rq, struct = task_struct *p) > } > } > =20 > -static void get_scan_cpumasks(cpumask_var_t cpus, struct task_struct *p) > -{ > -#ifdef CONFIG_NUMA_BALANCING > - int cpu, curr_cpu, nid, pref_nid; > - > - if (!static_branch_likely(&sched_numa_balancing)) > - goto out; > - > - cpu =3D READ_ONCE(p->mm->sc_stat.cpu); > - if (cpu !=3D -1) > - nid =3D cpu_to_node(cpu); > - curr_cpu =3D task_cpu(p); > - > - /* > - * Scanning in the preferred NUMA node is ideal. However, the NUMA > - * preferred node is per-task rather than per-process. It is possible > - * for different threads of the process to have distinct preferred > - * nodes; consequently, the process-wide preferred LLC may bounce > - * between different nodes. As a workaround, maintain the scan > - * CPU mask to also cover the process's current preferred LLC and the > - * current running node to mitigate the bouncing risk. > - * TBD: numa_group should be considered during task aggregation. > - */ > - pref_nid =3D p->numa_preferred_nid; > - /* honor the task's preferred node */ > - if (pref_nid =3D=3D NUMA_NO_NODE) > - goto out; > - > - cpumask_or(cpus, cpus, cpumask_of_node(pref_nid)); > - > - /* honor the task's preferred LLC CPU */ > - if (cpu !=3D -1 && !cpumask_test_cpu(cpu, cpus) && nid !=3D NUMA_NO_NOD= E) > - cpumask_or(cpus, cpus, cpumask_of_node(nid)); > - > - /* make sure the task's current running node is included */ > - if (!cpumask_test_cpu(curr_cpu, cpus)) > - cpumask_or(cpus, cpus, cpumask_of_node(cpu_to_node(curr_cpu))); > - > - return; > - > -out: > -#endif > - cpumask_copy(cpus, cpu_online_mask); > -} > - > static inline void update_avg_scale(u64 *avg, u64 sample) > { > int factor =3D per_cpu(sd_llc_size, raw_smp_processor_id()); > @@ -1866,7 +1835,18 @@ static void task_cache_work(struct callback_head *= work) > scoped_guard (cpus_read_lock) { > guard(rcu)(); > =20 > - get_scan_cpumasks(cpus, p); > + /* > + * Data race: While evaluating the visited_cpus without > + * a lock, a CPU could be concurrently set by > + * account_mm_sched(), meaning the scan might skip the newly > + * visited CPU if the bit changes during the scan. This is > + * a deliberate trade-off between accuracy and efficiency: > + * locking would prevent this race but incur extra overhead. > + * The missed runtime contribution is negligible because it > + * implies this process hasn't run on that CPU for a long > + * time, and will be captured in the next cycle. > + */ > + cpumask_and(cpus, cpu_online_mask, mm->sc_stat.visited_cpus); > =20 > for_each_cpu(cpu, cpus) { > /* XXX sched_cluster_active */ > @@ -1877,19 +1857,21 @@ static void task_cache_work(struct callback_head = *work) > if (!sd) > continue; > =20 > - for_each_cpu(i, sched_domain_span(sd)) { > - occ =3D fraction_mm_sched(cpu_rq(i), > - per_cpu_ptr(mm->sc_stat.pcpu_sched, i)); > + for_each_cpu_and(i, sched_domain_span(sd), mm->sc_stat.visited_cpus) = { > + cur =3D rcu_dereference_all(cpu_rq(i)->curr); > + if (cur && !(cur->flags & (PF_EXITING | PF_KTHREAD)) && > + cur->mm =3D=3D mm) > + nr_running++; > + > + occ =3D fraction_mm_sched(i, mm); > + if (occ =3D=3D 0) > + continue; > + > a_occ +=3D occ; > if (occ > m_occ) { > m_occ =3D occ; > m_cpu =3D i; > } > - > - cur =3D rcu_dereference_all(cpu_rq(i)->curr); > - if (cur && !(cur->flags & (PF_EXITING | PF_KTHREAD)) && > - cur->mm =3D=3D mm) > - nr_running++; > } > =20 > /*