From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (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 0E9FC4E3236; Thu, 17 Sep 2026 16:20:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789662029; cv=none; b=l5Z+zzmPjiAsAknoBjdC/YiThQWE2PZjffPlFMQa6SdxKWOR+DGU8vAe9VPAEDfCEfukDhZmkLBS4bUEHAay1bo3KjfqaaUr2h+jXsHbI5i+Kd0tsmmO2NeNRmgd5OJnHnvlKhm2p6s2k2vRzKgVwxNHz5Yt40+nQtVOYNqq0+0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789662029; c=relaxed/simple; bh=jWeA5fp+5Tks5UkmxcxrR2e8tFoG/jWgUbbfxnvpkD8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=dht3+NdBrIJi/EBG+ft0rMQfAx2VxsVKo55t/eX2AEsaQIk0sdqi+cIyyEGDEWurBqcskkD5gyjZfqKY42idc1bOQORJIeU/YxMwuUqXrcN2vmGZiF7pXJTuzp+LUodZSBzb1R3ezrsmgr7lYuHJSQbf13SgnSK6IOkXt6VVPH4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=pass smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=d8pRWvyF; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="d8pRWvyF" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=nb+0cojbEAyK8TGRBzLK8l83EOixpqcagNWfFDhOHak=; b=d8pRWvyFcOG/jj1FbSmrzWq965 B+9sYnTbXATIY8ZEWg1Jj/TVFjAY8Y0YQLAtNuIhYxyW7EHab1OaLSrqtI0fCRMmqE0gJ96q/53Vm 8Gjz8uj9+HQn2jeTqgVopG886BvnaKFKAGVwQqDuWDY3SsMPTcR9KMBYd4OSPQ38i8OcaDZn5GCxB 4IuitwTQyiJV5IsrDhdfvWlp4xPAS5Zy32HbTP9u5vM8mxRJbihDWaN6TN4WZoKomOY0bEOszyZ08 HbsxgHPKlTP4jy5TWSS9WCwhO+KNVVRFmKye+F8RuxH0/wXYkpk+H0zE9UxIWKg8erJj9iXQpxZHF lF29JHfw==; Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1x7Eq8-0000000B0Sy-3mMV; Thu, 17 Sep 2026 16:20:01 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id CBD72301BD5; Thu, 17 Sep 2026 18:19:57 +0200 (CEST) Date: Thu, 17 Sep 2026 18:19:57 +0200 From: Peter Zijlstra To: Gregory Price Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, kprateek.nayak@amd.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, usama.arif@linux.dev, kas@kernel.org, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, ying.huang@linux.alibaba.com, apopple@nvidia.com, jannh@google.com, pfalcato@suse.de, osalvador@suse.de, hannes@cmpxchg.org, raghavendra.kt@amd.com, stable@vger.kernel.org Subject: Re: [PATCH v2 4/4] sched/numa: do not let VMA PID activity gate promotion Message-ID: <20260917161957.GO4121339@noisy.programming.kicks-ass.net> References: <20260911001826.2109390-1-gourry@gourry.net> <20260911001826.2109390-5-gourry@gourry.net> 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=us-ascii Content-Disposition: inline In-Reply-To: <20260911001826.2109390-5-gourry@gourry.net> On Thu, Sep 10, 2026 at 08:18:26PM -0400, Gregory Price wrote: > From: "Gregory Price (Meta)" > > Commit fc137c0ddab2 ("sched/numa: enhance vma scanning logic") > skips VMAs without recent PID activity. Since only NUMA hint faults record > that activity, the filter can suppress the fault needed to promote hot > slow-tier memory. > > In tiering mode, scan PID-inactive VMAs using promotion-only scans. > Reevaluate this choice whenever the scanner visits a VMA and pass it with > each protection walk. > > Record socket-placement scans in prev_placement_scan_seq. Promotion-only > scans still update prev_scan_seq, but no longer postpone the starvation > fallback for placement scans. > > On a host with 768 GB of DRAM and 256 GB of CXL memory running two roughly > 430 GB database workloads, a large shmem VMA occupied each scan while 2,537 > other VMAs covering 84 GB were skipped as inactive. A hot 20 GB hash table > remained entirely on CXL before this change and was split evenly between > DRAM and CXL afterwards. > > Fixes: fc137c0ddab2 ("sched/numa: enhance vma scanning logic") > Cc: stable@vger.kernel.org > Assisted-by: LLM > Signed-off-by: Gregory Price (Meta) > --- > include/linux/mm_types.h | 7 ++++++ > kernel/sched/fair.c | 48 ++++++++++++++++++++++++++-------------- > 2 files changed, 39 insertions(+), 16 deletions(-) > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index 5413bd10fff2..9f042d6ad465 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -803,6 +803,13 @@ struct vma_numab_state { > * A VMA is not eligible for scanning if prev_scan_seq == numa_scan_seq > */ > int prev_scan_seq; > + > + /* > + * MM scan sequence ID when the VMA was last scanned for placement. > + * The starvation horizon in vma_is_accessed() counts against this, so > + * promotion-only scans cannot postpone placement indefinitely. > + */ > + int prev_placement_scan_seq; > }; > > #ifdef __HAVE_PFNMAP_TRACKING > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index e636e8de53f1..6d1da13a2ef5 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -4085,6 +4085,10 @@ static void reset_ptenuma_scan(struct task_struct *p) > p->mm->numa_scan_offset = 0; > } > > +/* > + * Decide whether this VMA should be sampled for NUMA placement. In addition > + * to recent accesses, periodically allow a scan to avoid starvation. > + */ > static bool vma_is_accessed(struct mm_struct *mm, struct vm_area_struct *vma) > { > unsigned long pids; > @@ -4101,22 +4105,13 @@ static bool vma_is_accessed(struct mm_struct *mm, struct vm_area_struct *vma) > if (test_bit(hash_32(current->pid, ilog2(BITS_PER_LONG)), &pids)) > return true; > > - /* > - * Complete a scan that has already started regardless of PID access, or > - * some VMAs may never be scanned in multi-threaded applications: > - */ > - if (mm->numa_scan_offset > vma->vm_start) { > - trace_sched_skip_vma_numa(mm, vma, NUMAB_SKIP_IGNORE_PID); > - return true; > - } > - > /* > * This vma has not been accessed for a while, and if the number > * the threads in the same process is low, which means no other > * threads can help scan this vma, force a vma scan. > */ > if (READ_ONCE(mm->numa_scan_seq) > > - (vma->numab_state->prev_scan_seq + get_nr_threads(current))) > + (vma->numab_state->prev_placement_scan_seq + get_nr_threads(current))) > return true; > > return false; > @@ -4142,7 +4137,7 @@ static void task_numa_work(struct callback_head *work) > unsigned int numab_mode = READ_ONCE(sysctl_numa_balancing_mode); > bool vma_pids_skipped; > bool vma_pids_forced = false; > - bool promo_only; > + bool accessed, scan_started, promo_only; > > WARN_ON_ONCE(p != container_of(work, struct task_struct, numa_work)); > > @@ -4278,6 +4273,7 @@ static void task_numa_work(struct callback_head *work) > * first scan: > */ > vma->numab_state->prev_scan_seq = mm->numa_scan_seq - 1; > + vma->numab_state->prev_placement_scan_seq = mm->numa_scan_seq - 1; > } > > /* > @@ -4307,17 +4303,30 @@ static void task_numa_work(struct callback_head *work) > } > > /* > - * Do not scan the VMA if task has not accessed it, unless no other > - * VMA candidate exists. > + * The PID filter must not gate promotion. Scan PID-inactive > + * VMAs in tiering mode using promotion-only scans. > */ > - if (!vma_pids_forced && !vma_is_accessed(mm, vma)) { > + accessed = vma_is_accessed(mm, vma); > + scan_started = mm->numa_scan_offset > vma->vm_start; > + > + if (!vma_pids_forced && !accessed && !scan_started && > + !(numab_mode & NUMA_BALANCING_MEMORY_TIERING)) { > vma_pids_skipped = true; > trace_sched_skip_vma_numa(mm, vma, NUMAB_SKIP_PID_INACTIVE); > continue; > } > + if (!vma_pids_forced && !accessed && > + !(numab_mode & NUMA_BALANCING_MEMORY_TIERING) && > + scan_started) > + trace_sched_skip_vma_numa(mm, vma, NUMAB_SKIP_IGNORE_PID); > > + /* > + * In combined mode, only VMAs the task uses need placement > + * samples. Without tiering, every VMA reaching here does. > + */ > promo_only = !(numab_mode & NUMA_BALANCING_NORMAL) || > - vma_is_ro_file(vma); > + ((numab_mode & NUMA_BALANCING_MEMORY_TIERING) && > + !accessed) || vma_is_ro_file(vma); > > do { > start = max(start, vma->vm_start); > @@ -4345,8 +4354,15 @@ static void task_numa_work(struct callback_head *work) > cond_resched(); > } while (end != vma->vm_end); > > - /* VMA scan is complete, do not scan until next sequence. */ > + /* > + * VMA scan is complete, do not scan until next sequence. A > + * promotion-only scan reached the end of the VMA but did not > + * sample placement, so it does not count towards the starvation > + * horizon in vma_is_accessed(). > + */ > vma->numab_state->prev_scan_seq = mm->numa_scan_seq; > + if (!promo_only) > + vma->numab_state->prev_placement_scan_seq = mm->numa_scan_seq; > > /* > * Only force scan within one VMA at a time, to limit the Not a fan of what that tiering code is causing :/ But I suppose this will do; Mel?