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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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260911001826.2109390-5-gourry@gourry.net> X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: E91D010000B X-Stat-Signature: xngpqzubt9777szjmyxuk7m8cpyp867d X-Rspam-User: X-HE-Tag: 1789662009-407844 X-HE-Meta: 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 +vWt04S/ pP7TpGM6apuoD5k0hbA2xVNSWAfalcAKHHGJwiBFnHoAr250wg73XSg+BUDAL7VP+fVuPkFdwm16561SXraUs3rwuZUMFvvUas7EbP4oi7nWdmaY+d/Fyh/CxpMU0kK/LknVhqj6jApjyfKiS1rawZqJria5jm3D91VQcCqyWMXj05k3DrOfTI5RVMSX2pFGeRFojaBHU80sEGUq9IxwlmAib6iOwxnl+qEBeidDo5mS0PM8nXxjesPfHlZOShxd0K0JTbFd2iQ3obqQ2l5esgsvd20l6eEKSrjScXrQEplv5fiWGJzgc8vC1noOzTNA4L/1kAzULxX5JfS6pB+krrpciAQv40wWXKIyLpzxqiBtGUo4= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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?