From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id D4BD839FD0 for ; Mon, 1 Jul 2024 11:47:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719834425; cv=none; b=IwJjwmOKbyZWFtBu81AM3dbFUwiE75LKiwC4UkRBMDSPQlg4GPbvX+Q9DCTQISzGzUfE0nbqSZG0VX7exlRF7+BkaP6QkjoRagdc5oJTHjXbjaMOnlj0OAoQQX6A6mdx6ZeYrre8FagcH+jIsqr4B6QG3z7KY5MVaZ5f8mrvWyI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719834425; c=relaxed/simple; bh=QbMeO0zCEIjsw7BElAH3ADx8DVUzqr7ztJ+td4iFcjY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=GZyJZHUV6KhxuNiOUTk0kCQnXreb0K3v6cY/ITuZXKdIAhf+Ez7P8Cq48l30mUD7lJACcV3hvuqsGzFEuMl7fyCOmqIGe2E3zogtuiy0+09+gDDSnKu7CiMXQ4LAbboy90FtOJ9BJ877aOief4+Fr5vOZ6IqyuhC2yCeNI1p2Wk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id AC80B339; Mon, 1 Jul 2024 04:47:26 -0700 (PDT) Received: from [10.57.72.41] (unknown [10.57.72.41]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 1A8F63F762; Mon, 1 Jul 2024 04:46:58 -0700 (PDT) Message-ID: <9bbbf8be-a197-476d-b491-92c9e31e8119@arm.com> Date: Mon, 1 Jul 2024 12:46:57 +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 v2 2/2] mm: add docs for per-order mTHP split counters Content-Language: en-GB To: Lance Yang Cc: akpm@linux-foundation.org, dj456119@gmail.com, 21cnbao@gmail.com, david@redhat.com, shy828301@gmail.com, ziy@nvidia.com, libang.li@antgroup.com, baolin.wang@linux.alibaba.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Mingzhe Yang References: <20240628130750.73097-1-ioworker0@gmail.com> <20240628130750.73097-3-ioworker0@gmail.com> <13dcf4be-8c5f-4697-adc1-b68c3da82d78@arm.com> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 01/07/2024 11:50, Lance Yang wrote: > On Mon, Jul 1, 2024 at 4:31 PM Ryan Roberts wrote: >> >> On 28/06/2024 14:07, Lance Yang wrote: >>> This commit introduces documentation for mTHP split counters in >>> transhuge.rst. >>> >>> Signed-off-by: Mingzhe Yang >>> Signed-off-by: Lance Yang >>> --- >>> Documentation/admin-guide/mm/transhuge.rst | 16 ++++++++++++++++ >>> 1 file changed, 16 insertions(+) >>> >>> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst >>> index 1f72b00af5d3..709fe10b60f4 100644 >>> --- a/Documentation/admin-guide/mm/transhuge.rst >>> +++ b/Documentation/admin-guide/mm/transhuge.rst >>> @@ -514,6 +514,22 @@ file_fallback_charge >>> falls back to using small pages even though the allocation was >>> successful. >> >> >> I note at the top of this section there is a note: >> >> Monitoring usage >> ================ >> >> .. note:: >> Currently the below counters only record events relating to >> PMD-sized THP. Events relating to other THP sizes are not included. >> >> Which is out of date, now that we support mTHP stats. Perhaps it should be removed? > > Good catch! Let's remove that in this patch ;) > >> >>> >>> +split >>> + is incremented every time a huge page is successfully split into >>> + base pages. This can happen for a variety of reasons but a common >>> + reason is that a huge page is old and is being reclaimed. >>> + This action implies splitting any block mappings into PTEs. >> >> Now that I'm reading this, I'm reminded that Yang Shi suggested at LSFMM that a >> potential aid so solving the swap-out fragmentation problem is to split high >> orders to lower (but not 0) orders. I don't know if we would take that route, >> but in principle it sounds like splitting mTHP to smaller mTHP might be >> something we want some day. I wonder if we should spec this counter to also >> include splits to smaller orders and not just splits to base pages? >> >> Actually looking at the code, I think split_huge_page_to_list_to_order(order>0) >> would already increment this counter without actually splitting to base pages. >> So the documantation should probably just reflect that. > > Yep, you're right. > > It’s important that the documentation reflects that to ensure consistency. > > How about "... is successfully split into smaller orders. This can..."? fine by me. > > Thanks, > Lance > >> >>> + >>> +split_failed >>> + is incremented if kernel fails to split huge >>> + page. This can happen if the page was pinned by somebody. >>> + >>> +split_deferred >>> + is incremented when a huge page is put onto split >>> + queue. This happens when a huge page is partially unmapped and >>> + splitting it would free up some memory. Pages on split queue are >>> + going to be split under memory pressure. >>> + >>> As the system ages, allocating huge pages may be expensive as the >>> system uses memory compaction to copy data around memory to free a >>> huge page for use. There are some counters in ``/proc/vmstat`` to help >>