From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 4CB4A34D909 for ; Wed, 14 Jan 2026 11:40:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768390854; cv=none; b=BBjVLwYsuHdIUPCRLCPdcVw6szEBUblxwBBecUqgk6T3U1Q6JjcKcT+/YxWAH9iKesLmncEJMCFWL7tgfXBvqf1xwVywy5o7Ewj5kLh143GcxiDZnSoniNeUTdNgxGj7dVZW7CkjGnAXlwJT24V5JG1sfLx/Sbg0Mq3zFNaMmqA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768390854; c=relaxed/simple; bh=dP+lw/+mW6tC9mSGsXiahpdhcfiBeFh8hQKPJymjUzU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=gXLJ06RuIzyTDSZvDL0mEX6/xxyF0kJWCcEy8JxsIK7NPd1GXn7vQT8xusS6FU5qBfJcj89J+2LRXplW7B0TvLTZsOtQTs9GgrN4hOdN6I6s5x7D9NGS3fU0GR4shgdM5B6AAiMUu2jCxCwlNLOqC4sckdIlpx/MIDlgNsi3jv0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=EnISaw+M; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="EnISaw+M" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AD01DC4CEF7; Wed, 14 Jan 2026 11:40:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768390853; bh=dP+lw/+mW6tC9mSGsXiahpdhcfiBeFh8hQKPJymjUzU=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=EnISaw+MJcidIWBxl3b2zyOxLiLqneya2LRIly1pSz+adbT+hIsMBszDoq/1XxQzT EOwmS08G5yDnzw3r7FRUZbwaLNcO/rA2YGJYEh6jGR7CihhTNJ3W5qnRq9k8bhEE0f w+Zj/ndDQj8tzdfVTWrkrIpIj6lgzt8m3oN54PgePS0YgN3ZLCtqEEYtP1dVZmviF+ GXwlHxwRsG6SIi1jjZ7Hb+inHCPLpl8RCg4taWSRR2ZmA+H6GBWHVBVnnPKxhFs8z3 3pc/+pqwE7BOkwJMB53XTwf6BAlt0meA22RFzZYte+rzecCVAlxWKeH6lZ0/orHghG v+pgANAGokrGQ== Message-ID: <9924fc6c-e3ad-422a-ad60-756efccba0aa@kernel.org> Date: Wed, 14 Jan 2026 12:40:50 +0100 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 mm-new v4 3/6] mm: khugepaged: just skip when the memory has been collapsed To: Vernon Yang , akpm@linux-foundation.org Cc: lorenzo.stoakes@oracle.com, ziy@nvidia.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Vernon Yang References: <20260111121909.8410-1-yanglincheng@kylinos.cn> <20260111121909.8410-4-yanglincheng@kylinos.cn> From: "David Hildenbrand (Red Hat)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 1/11/26 13:19, Vernon Yang wrote: > The following data is traced by bpftrace on a desktop system. After > the system has been left idle for 10 minutes upon booting, a lot of > SCAN_PMD_MAPPED or SCAN_NO_PTE_TABLE are observed during a full scan > by khugepaged. > > @scan_pmd_status[1]: 1 ## SCAN_SUCCEED > @scan_pmd_status[6]: 2 ## SCAN_EXCEED_SHARED_PTE > @scan_pmd_status[3]: 142 ## SCAN_PMD_MAPPED > @scan_pmd_status[2]: 178 ## SCAN_NO_PTE_TABLE > total progress size: 674 MB > Total time : 419 seconds ## include khugepaged_scan_sleep_millisecs > > The khugepaged_scan list save all task that support collapse into hugepage, > as long as the task is not destroyed, khugepaged will not remove it from > the khugepaged_scan list. This exist a phenomenon where task has already > collapsed all memory regions into hugepage, but khugepaged continues to > scan it, which wastes CPU time and invalid, and due to > khugepaged_scan_sleep_millisecs (default 10s) causes a long wait for > scanning a large number of invalid task, so scanning really valid task > is later. > > After applying this patch, when the memory is either SCAN_PMD_MAPPED or > SCAN_NO_PTE_TABLE, just skip it, as follow: > > @scan_pmd_status[6]: 2 > @scan_pmd_status[3]: 147 > @scan_pmd_status[2]: 173 > total progress size: 45 MB > Total time : 20 seconds > > Signed-off-by: Vernon Yang > --- > mm/khugepaged.c | 17 +++++++++++------ > 1 file changed, 11 insertions(+), 6 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 5c6015ac7b5e..6df2857d94c6 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -68,7 +68,10 @@ enum scan_result { > static struct task_struct *khugepaged_thread __read_mostly; > static DEFINE_MUTEX(khugepaged_mutex); > > -/* default scan 8*HPAGE_PMD_NR ptes (or vmas) every 10 second */ > +/* > + * default scan 8*HPAGE_PMD_NR ptes, pmd_mapped, no_pte_table or vmas > + * every 10 second. > + */ > static unsigned int khugepaged_pages_to_scan __read_mostly; > static unsigned int khugepaged_pages_collapsed; > static unsigned int khugepaged_full_scans; > @@ -1267,7 +1270,7 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm, > result = find_pmd_or_thp_or_none(mm, start_addr, &pmd); > if (result != SCAN_SUCCEED) { > if (cur_progress) > - *cur_progress = HPAGE_PMD_NR; > + *cur_progress = 1; > goto out; > } > > @@ -1276,7 +1279,7 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm, > pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl); > if (!pte) { > if (cur_progress) > - *cur_progress = HPAGE_PMD_NR; > + *cur_progress = 1; > result = SCAN_NO_PTE_TABLE; > goto out; > } The above checks are clear. > @@ -2347,9 +2350,6 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned > continue; > } > > - if (cur_progress) > - *cur_progress += folio_nr_pages(folio); > - This is a all a bit hairy. Assume we found a single 4k folio in the xarray, but then collapse a 2M THP. Is the progress really "1" ? What about shmem swap entries (xa_is_value)? So I think the whole file path needs more thought -- Cheers David