From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 C45313E51DE; Fri, 25 Sep 2026 10:55:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790333729; cv=none; b=DX02jrbdD4UrpzvzH4ZcSfdc4S4s4zK4omf9sLw26H7+aA6WvVeZyoF9J1RUOLxiL3MuJycosmrMjIFZ5grs1FEp5bdgAMAjedZw/71LgjcIYoJ5+qD8BiQkffQJTWHN6IclcSe1xX8IWTHxUkFhhl2l1k6oHcRhCTB8OqjNi50= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790333729; c=relaxed/simple; bh=90oCiblxcmMKki320b5oH8g1qw2hyPccBSLwPjfnkIw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=rFeDrPFBGybRew0xJYUROnmwCATsN7VX5q/4HCybOJeWUPQrCUbbdt0NFBqyJp9xNmXyK+t2D0h6itytaCcnjZc5noPRLrQvO54yoijkh2JV5UnwWDt9G8UyuBv9KqGcm9v0ifJKYaWRhU69QD/gW12tXNyNR3SQXE+rmyrCcHw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=konjO3sD; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="konjO3sD" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3820D1F000FF; Fri, 25 Sep 2026 10:55:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790333722; bh=ATppPA08cbrd3rkkrcMCwx3kOQ21INX+r6Ut92RHrGc=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=konjO3sD23vveaJBinGM0SJByCg78KytyAQj8Pht9Dv5oz1t2mIBzNIe/XRlEdNJX XnpYp27AgOerWofLyjYuQe4vlt30/DNLVeUbBBdPvonILl3DS1TXLEzSRF4EbYgTFS mauWH0jz4ZCRYic2poGKUz4ee2pfoawHcb/fOhaunODCqLpsf9fbmfGTMa3MG1YlJV Cm5/yDYv/IWyPnQ5MPTBmWji5IYL783VGrNjFn3Hb5FT/BVw0zf0xLxYVsWmKKJOWD U23iyusaUTu7lwNj+0RdCqBlMpkDq370R2ueOlaYwhirmA41+ru+ohVYwOhqf0paF1 cWczDrPzDVMjw== Message-ID: Date: Fri, 25 Sep 2026 12:55:09 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 5/7] sched/numa: scan PID-inactive VMAs for promotion To: Gregory Price , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, akpm@linux-foundation.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, mingo@redhat.com, peterz@infradead.org, 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, hannes@cmpxchg.org, shy828301@gmail.com, raghavendra.kt@amd.com, stable@vger.kernel.org References: <20260922182928.2199090-1-gourry@gourry.net> <20260922182928.2199090-6-gourry@gourry.net> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/22/26 20:29, 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. > > Let memory tiering bypass the PID scan filter. In combined mode, use the > placement decision to restrict top-tier sampling to VMAs that need it, > while inactive VMAs still receive promotion-only scans. > > Track the last completed placement scan separately from scans of any > kind. Promotion-only scans still update prev_scan_seq, but do not > advance the placement-starvation horizon. > > On a host with 768 GB of DRAM and 256 GB of CXL memory, one large shmem > VMA consumed most scanning activity, while 2,537 other VMAs covering > 84 GB were skipped as inactive. One stand-out result: a hot 20 GB hash > table ended up trapped entirely on CXL and drove CXL bandwidth > utilization beyond sustainable levels - resulting in a large regression. > > With this series, the hot hash table ends up split evenly between DRAM > and CXL, tier residency tracked runtime load, and CXL bandwidth > utilization drops from 45GB/s (maxed) to 5-10GB/s, while DRAM bandwidth > utilization increases from ~200GB/s to 250GB/s+, resulting in major > throughput improvements for the database workload. > > Fixes: fc137c0ddab2 ("sched/numa: enhance vma scanning logic") > Cc: stable@vger.kernel.org > Assisted-by: OpenAI:gpt-5 > Signed-off-by: Gregory Price (Meta) > --- > include/linux/mm_types.h | 7 +++++++ > kernel/sched/fair.c | 26 +++++++++++++++++++++----- > 2 files changed, 28 insertions(+), 5 deletions(-) > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index fd35db969bc94..dcca3ead9db59 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -804,6 +804,13 @@ struct vma_numab_state { > */ > int prev_scan_seq; > > + /* > + * MM scan sequence ID when the VMA was last scanned for placement. > + * The starvation horizon in vma_needs_placement_scan() counts against > + * this, so promotion-only scans cannot postpone placement indefinitely. > + */ > + int prev_placement_scan_seq; > + > /* Preserve placement-scan eligibility during an in-progress scan. */ > bool placement_scan; > }; > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index a2849e72c4e26..412c72084a63d 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -4097,7 +4097,7 @@ static bool vma_needs_placement_scan(struct mm_struct *mm, > * 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; > @@ -4267,7 +4267,8 @@ static void task_numa_work(struct callback_head *work) > * to prevent VMAs being skipped prematurely on the > * first scan: > */ > - vma->numab_state->prev_scan_seq = mm->numa_scan_seq - 1; > + vma->numab_state->prev_scan_seq = mm->numa_scan_seq - 1; > + vma->numab_state->prev_placement_scan_seq = mm->numa_scan_seq - 1; > } > > /* > @@ -4300,10 +4301,13 @@ static void task_numa_work(struct callback_head *work) > * Do not scan the VMA if a task has not accessed it, unless no other > * VMA candidate exists. If a scan is already in-progress, finish it, > * but track continuation separately from starting a new one. > + * > + * The PID filter must not gate promotion. Allow PID-inactive VMAs > + * to proceed when memory tiering is enabled. > */ > placement_due = vma_needs_placement_scan(mm, vma); > scan_started = mm->numa_scan_offset > vma->vm_start; > - pid_scan_allowed = vma_pids_forced || placement_due; > + pid_scan_allowed = tiering || vma_pids_forced || placement_due; > > if (!pid_scan_allowed) { > if (scan_started) { > @@ -4315,10 +4319,16 @@ static void task_numa_work(struct callback_head *work) > } > } > > - /* Keep scan policy stable while processing a VMA in chunks.*/ > + /* > + * Keep scan policy stable while processing a VMA in chunks. > + * A fault in one chunk can make a VMA placement-eligible. Keep a > + * promotion-only decision sticky for the rest of a partial scan. > + */ > placement_scan &= numab_mode & NUMA_BALANCING_NORMAL; > if (scan_started) > placement_scan &= vma->numab_state->placement_scan; > + else if (tiering) > + placement_scan &= placement_due; Same comment. Apart from that LGTM. -- Cheers, David