From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-177.mta0.migadu.com (out-177.mta0.migadu.com [91.218.175.177]) (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 76C3F314B62 for ; Mon, 5 Jan 2026 03:36:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.177 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767584177; cv=none; b=q6MSJf/84oMNBTknLthiah/ki6SLGpWmj+roTXW9rEblDuvAVPgHHDpz94jgrXqBEwtGFOB9MK9ZyFkWzroNIR/+nDSXvZkYtQljHUJe+7s4VJ/KDFMZ3FPfT/Wnp868v6E8Py4TWGZFKSuF1w6hBjT+rmna3y8+B4suWdbpOxA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767584177; c=relaxed/simple; bh=3MsR4+efn3PdQWjQb0THU1v7+50iEbkXU8tyqB2GaQg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=YhzPY+xrlU9kZDSgrR4FkCTzFTaMUa+zC8j/5zFxD9X1YHCDZwFgMpNCwZVortIdmKb+5QbtfvpwOJzvzI7jwqs1uf2Q2Z2iv7nLF1NyMKwzA70ee5K37etrYldF9rTuHuFm09Bb6WgH3OB+WfgmylQCVHYQpes5/jXSFd9djJU= 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=qYedAKQ6; arc=none smtp.client-ip=91.218.175.177 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="qYedAKQ6" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1767584166; 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=YlCcjoq+l3rFwtJec/nP8H1oFR5IX71pefYD83vbjnE=; b=qYedAKQ65tFmyTfpB3G85TRc81oYXWEkUz1/zrsUf/Xz0ark+Ha4sOGRd+3uDO/lD76rMt UxJuLmX/JUuHucnLTQ0QNIUe+kUsJX9al//G8FhUqsinT+/NbSchXzzuxPeQ1a9StNjH3G ktKmMu63f2v4yqZOgvQejR3lhrMpgz4= Date: Mon, 5 Jan 2026 11:35:58 +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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2026/1/5 11:12, Vernon Yang wrote: > On Mon, Jan 5, 2026 at 10:51 AM Lance Yang wrote: >> >> 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. > > Same above, the lazyfree folios will be set as non-lazyfree Nop ... > in the memory reclaim path, it is not skipped in the next scan, > the PMD region will collapse :) Let me be more specific: Assume we have a PMD region (512 pages): - Pages 0-499: hot pages (frequently accessed, NOT lazyfree) - Pages 500-511: lazyfree pages (MADV_FREE'd and clean) This patch skips the entire region when it hits page 500. So pages 0-499 can't be collapsed, even though they are hot. I'm NOT saying lazyfree pages themselves become hot ;) As I mentioned earlier, 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 ... > >>> >>>> 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 >>