From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-188.mta1.migadu.com (out-188.mta1.migadu.com [95.215.58.188]) (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 255B52AE99 for ; Mon, 5 Jan 2026 02:51:24 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.188 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767581488; cv=none; b=alZhwlle23RlL5Qrh0aCk4SbujVZ0tivRaE117tOv+urMVA2zo5MpXUquQA+S6C6RaLjy0xJcXsQaubiWYPtfZGPk2ygsYAzo24D8yDSnSF54bDpGTRf2I10HFcLYXwe5Mk41WAeWyJYNbcGJKca2Ud5LPSDj6ZKrwUATD2ROQk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767581488; c=relaxed/simple; bh=//KHOHjZeva7sHbJYNvl5n01ZpgIGSNXiLEHO8hOXCI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ncIio87MleGLqhjFuxtBI24CWypWIMK8EanoZADdqQFvf2qpnszVJ6DeZkjdVDv8HY7y62hDXbIsjomfjHn1n7iO0HSczlwWqYtJgYem6ygWNi4O55CjgQB69xQ62C3PIe2q++Hqsk9q43DU92pPN7swGWRPx8CnIi76dcDtjp4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=i3pzTf5e; arc=none smtp.client-ip=95.215.58.188 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="i3pzTf5e" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1767581483; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=eH363o8l7dNNtRLISLiXPZUzX0hpm1ZZb6O7fLroI1E=; b=i3pzTf5edwAE3CTPa9VtqIBKVOIlYcyOpJPjezhcPUSu7xjSs6+BMw/g/BO0T/YxVkDHhD sFW4mvIrEl2oxn/kwyYeIH3lWhCslJisSAfFaOKzJfB+tqEL/nrHhMCJyqszlA3XuNPQzN ax4yT25TMFOxSTV++rMxnFxiNBuxCQE= Date: Mon, 5 Jan 2026 10:51:09 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v3 5/6] mm: khugepaged: skip lazy-free folios at scanning Content-Language: en-US To: Vernon Yang Cc: lorenzo.stoakes@oracle.com, ziy@nvidia.com, dev.jain@arm.com, baohua@kernel.org, richard.weiyang@gmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Vernon Yang , akpm@linux-foundation.org, david@kernel.org References: <20260104054112.4541-1-yanglincheng@kylinos.cn> <20260104054112.4541-6-yanglincheng@kylinos.cn> <9c82ffaa-5f62-4110-80cc-00f0c46e90fb@linux.dev> <3lbptab7e2nhqilwnoccq6kxks2r55j3ffqtslt62o2qtgulk5@w4mwglb2kd75> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: <3lbptab7e2nhqilwnoccq6kxks2r55j3ffqtslt62o2qtgulk5@w4mwglb2kd75> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 2026/1/5 09:48, Vernon Yang wrote: > On Sun, Jan 04, 2026 at 08:10:17PM +0800, Lance Yang wrote: >> >> >> On 2026/1/4 13:41, Vernon Yang wrote: >>> For example, create three task: hot1 -> cold -> hot2. After all three >>> task are created, each allocate memory 128MB. the hot1/hot2 task >>> continuously access 128 MB memory, while the cold task only accesses >>> its memory briefly andthen call madvise(MADV_FREE). However, khugepaged >>> still prioritizes scanning the cold task and only scans the hot2 task >>> after completing the scan of the cold task. >>> >>> So if the user has explicitly informed us via MADV_FREE that this memory >>> will be freed, it is appropriate for khugepaged to skip it only, thereby >>> avoiding unnecessary scan and collapse operations to reducing CPU >>> wastage. >>> >>> Here are the performance test results: >>> (Throughput bigger is better, other smaller is better) >>> >>> Testing on x86_64 machine: >>> >>> | task hot2 | without patch | with patch | delta | >>> |---------------------|---------------|---------------|---------| >>> | total accesses time | 3.14 sec | 2.93 sec | -6.69% | >>> | cycles per access | 4.96 | 2.21 | -55.44% | >>> | Throughput | 104.38 M/sec | 111.89 M/sec | +7.19% | >>> | dTLB-load-misses | 284814532 | 69597236 | -75.56% | >>> >>> Testing on qemu-system-x86_64 -enable-kvm: >>> >>> | task hot2 | without patch | with patch | delta | >>> |---------------------|---------------|---------------|---------| >>> | total accesses time | 3.35 sec | 2.96 sec | -11.64% | >>> | cycles per access | 7.29 | 2.07 | -71.60% | >>> | Throughput | 97.67 M/sec | 110.77 M/sec | +13.41% | >>> | dTLB-load-misses | 241600871 | 3216108 | -98.67% | >>> >>> Signed-off-by: Vernon Yang >>> --- >>> include/trace/events/huge_memory.h | 1 + >>> mm/khugepaged.c | 6 ++++++ >>> 2 files changed, 7 insertions(+) >>> >>> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h >>> index 01225dd27ad5..e99d5f71f2a4 100644 >>> --- a/include/trace/events/huge_memory.h >>> +++ b/include/trace/events/huge_memory.h >>> @@ -25,6 +25,7 @@ >>> EM( SCAN_PAGE_LRU, "page_not_in_lru") \ >>> EM( SCAN_PAGE_LOCK, "page_locked") \ >>> EM( SCAN_PAGE_ANON, "page_not_anon") \ >>> + EM( SCAN_PAGE_LAZYFREE, "page_lazyfree") \ >>> EM( SCAN_PAGE_COMPOUND, "page_compound") \ >>> EM( SCAN_ANY_PROCESS, "no_process_for_page") \ >>> EM( SCAN_VMA_NULL, "vma_null") \ >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>> index 30786c706c4a..1ca034a5f653 100644 >>> --- a/mm/khugepaged.c >>> +++ b/mm/khugepaged.c >>> @@ -45,6 +45,7 @@ enum scan_result { >>> SCAN_PAGE_LRU, >>> SCAN_PAGE_LOCK, >>> SCAN_PAGE_ANON, >>> + SCAN_PAGE_LAZYFREE, >>> SCAN_PAGE_COMPOUND, >>> SCAN_ANY_PROCESS, >>> SCAN_VMA_NULL, >>> @@ -1337,6 +1338,11 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm, >>> } >>> folio = page_folio(page); >>> + if (folio_is_lazyfree(folio)) { >>> + result = SCAN_PAGE_LAZYFREE; >>> + goto out_unmap; >>> + } >> >> That's a bit tricky ... I don't think we need to handle MADV_FREE pages >> differently :) >> >> MADV_FREE pages are likely cold memory, but what if there are just >> a few MADV_FREE pages in a hot memory region? Skipping the entire >> region would be unfortunate ... > > If there are hot in lazyfree folios, the folio will be set as non-lazyfree > in the memory reclaim path, it is not skipped in the next scan in the > khugepaged. > > shrink_folio_list() > try_to_unmap() > folio_set_swapbacked() > > If there are no hot in lazyfree folios, continuing the collapse would > waste CPU and require a long wait (khugepaged_scan_sleep_millisecs). > Additionally, due to collapse hugepage become non-lazyfree, preventing > the rapid release of lazyfree folios in the memory reclaim path. > > So skipping lazy-free folios make sense here for us. > > If I missed something, please let me know, thank! I'm not saying lazyfree pages become hot :) If a PMD region has mostly hot pages but just a few lazyfree pages, we would skip the entire region. Those hot pages won't be collapsed. > >> Also, even if we skip these pages now, after they are reclaimed, they >> become pte_none. Then khugepaged will try to collapse them anyway >> (based on khugepaged_max_ptes_none). So skipping them just delays >> things, it does not really change the final result ;) > > This patch just resolve scene for hot1 -> cold -> hot2. > > -- > Thanks, > Vernon