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 AECAD5616C0; Tue, 22 Sep 2026 22:13:02 +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=1790115193; cv=none; b=a6UA/alCL56bc/bOKvPOaJmERfbatX1Olfi85R5jNW2qxbKQNn+CcZgz6lh3l5ufhP+buVv5bj0HoItG5nnRpTOM1wLUtmLqpV6YyAtQ6q4QK1/+9y5Jcp+tZwlThUHEt/1IOMOeIdqFQuoD9P4p4B0vys0uuAgWsiCwVMW8Rx8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790115193; c=relaxed/simple; bh=i5QH9OtSe6Mko3XfT6VsMrUIuOag608CTs7juOYk2x0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=fBOWsfJQTAf0up7ZgV+VR6qF/mQkkRpvd5YV24WlK21clEBpg7Uq9g9RYouHWhwnYbplQ5aSnKUhjWxcFig+Til//D9S+9rbSHDDm/gQdm9Hby5vo8ImFp8sh8Lzx/74arXC8fcU97l0z2u4jVIi3BYNVQH6032ZYscXgGm0Zuc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=h/oEr1hD; 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="h/oEr1hD" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D257E1F000FF; Tue, 22 Sep 2026 22:12:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790115175; bh=hOhEGXrCm5RukHPz+l9coDc0fzxvUt1JOQ5e3Wz87fw=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=h/oEr1hDSb1mAHEjy4u94E7otCerB91QM1iBzCWksRAZBl+L9Mmd7Z9HdhqMSHenP gq2EDNXyAV6Mj+hwHR3ucLLg6HPSIAny23UeIcYAYop5+q/jyjNOVmpri+PqQtcmXZ Y/aTFryO8bBfKMJEIxA2o7zmWogRX4fbcneIk3KU6foS7PVVLmPwoD5D3gJ3VBa5CA wCn/8dDfjsfviCjegkLZq7BHx/dJCoArtbKsDRxkJNEC2CiykJrIgWbSpgEVOjYO0S 74J+/0SoqldJDYFLVgI26GOTBIbuI2NmHeg1mI3HFIm5YN7JicZFPENiu2YzbARMU+ qlcWf5U1t8FDA== Date: Wed, 23 Sep 2026 08:12:46 +1000 From: Dave Chinner To: MingTao Huang <1037827920@qq.com> Cc: Carlos Maiolino , Dave Chinner , "Darrick J . Wong" , Chandan Babu R , linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org, MingTao Huang Subject: Re: [PATCH] xfs: fix skipped flushing items not counted in xfsaild_push() Message-ID: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Sep 22, 2026 at 04:14:29PM +0800, MingTao Huang wrote: > From: MingTao Huang > > Commit f3f7ae68a4ea ("xfs: skip flushing log items during push") > introduced a fast path in xfsaild_push() that uses > test_bit(XFS_LI_FLUSHING) to skip log items already being > flushed. However, the fast path jumps directly to the > next_item label, bypassing flushing++, count++, and the > ail_last_pushed_lsn update. This causes three problems: Right, they get skipped because they are already flushing and we are not doing anything with them. i.e. they've already been accounted as flushing (e.g. inodes that were gathered into a single buffer flush by xfs_iflush_cluster()) and the buffer is already either on the delwri list for submission or under IO. > > 1. The loop exit condition "count > 1000" becomes much harder to > trigger. count was meant to track every item visited, but now it No it isn't. As the author of this code, I can say for certain that the intent of 'count' is to count the number of items we made pushing decisions about, not count the number of items we have iterated. The purpose of 'flushing' is to account for the number of buffers we accumulate on the delwri list before we submit it. As count is incremented whenever flushing is incremented, it currently forms an upper bound to the number of buffers that can be queued on the delwri list in a single push iteration. The purpose of 'stuck' is to account for items that could not be placed on the delwri list because they were locked, pinned or otherwise unavailable for flushing. 'success' is not directly counted, because there is no further action that needs to be taken for them; they are simply counted, and hence contribute to the flush/stuck ratio decisions later. IOWs, every 'push decision' that could add a buffer to the delwri list is accounted by 'count', and flushing/stuck indicate what decision was made. The point of LI_FLUSHING was simply to skip over all the items that would not change either count, flushing or stuck because they have already been accounted to 'count' and 'flushing' by a prior push decision. > only increments for non-flushing items that enter > xfsaild_push_item(). Exactly - count is the number of items we made new pushing decsions about. > Each such item typically triggers an inode > cluster flush that marks dozens of neighbouring inodes as flushing, > so count effectively counts cluster flushes rather than individual > items. Yes, that is exactly the intent - a single cluster buffer on the delwri list covers up to 32 inode items on the AIL. That is -one IO- for up to 32 items on the AIL, and it is -IO- that we are trying to account for and optimise here, not log items. > The threshold shifts from 1000 items to ~1000 clusters, No, the count has always been '1000 new buffer IO decisions made', not '1000 items processed'. > letting the loop scan an order of magnitude more items per round. > Each cluster flush adds a buffer to ail_buf_list, and the resulting > oversized list causes xfs_buf_delwri_submit_nowait() -- which runs > list_sort() plus per-buffer trylock and IO submission -- to take so > long that the watchdog fires. How? count bounds the delwri list to 1000 buffers at most. That's very intentional, and the number is based on benchmarking results. i.e. A thousand items on a list takes a couple of milliseconds to sort. I/O submission of that list takes a couple of milliseconds if it doesn't block (i.e. the IO subsystem has to be faster than the CPU IO submission loop to avoid block on a full queue), and if it does block, we want all the IO submission to merge as effectively as possible in the queue so that we use as few IO slots in the queue as possible. Hence the submission code should not be running around in a hard loop for 22s because of a list that is too long. The buffer list length is -bounded- by count, and that should prevent these sorts of issues. > 2. The timeout decision "(stuck + flushing) * 100 / count > 90" is > computed without the fast-path flushing items, so the flushing ratio > is severely under-reported. No, the accounting this check is acting on is correct. The action we take is based on the all the items we tried to push this iteration, not the indirect LI_FLUSHING items that we didn't make a decision about or are still in progress from the previous iteration. > When most of the AIL is flushing, the > ratio appears near 0%. How did you get into a state where most of the AIL is flushing and no progress is being made transitioning items out of flushing state? i.e. if delwri submission is not making progress, then the ail push will keep appending to the delwri list (by design, so we retry flushing items until submission succeeds). This will result in transitioning most of the AIL to flushing state and queued on the delwri list. This is a symptom of the delwri submission not making progress and rather than a problem with AIL flushing accounting. i.e. we need to know why delwri submission is not making progress, as that is likely the real cause of the problem. i.e. it is unlikely that this has anythign to do with how we account for item push decisions, and it is likely your changes just slow down or change the timing of item pushing sufficiently to avoid whatever issue is occuring in delwri submission. > xfsaild therefore selects > "tout = 0" when it should select "tout = 20" (20 ms > back-off to let IO complete). The zero-backoff tight loop compounds > the ail_buf_list accumulation across rounds. > > 3. ail_last_pushed_lsn is not advanced past flushing items, so the > next push round restarts scanning from the same position, repeatedly > traversing items that are still in-flight. This is why we do not account for flushing items that we skip. It will walk over them as fast as possible until it reaches items that it can push. Those items will then be accounted as flushing/stuck/count. This will then move the last_lsn forwards appropriately or trigger a log force or backoff sleep and so it should not get stuck spinning for 22s in that case, either. > We hit this as a soft lockup during stress testing on an internal > kernel that includes commit f3f7ae68a4ea ("xfs: skip flushing log > items during push"). The xfsaild kthread was stuck for > 22 seconds inside xfs_buf_delwri_submit_nowait(), called from > xfsaild_push(), processing an excessively large ail_buf_list: Tell me how the delwri list got so long that it gets stuck inside xfs_buf_delwri_submit_nowait(). Whatever caused that is the bug that we need to understand and fix - changing accounting to make gross behavioural changes that result in exceedingly inefficient CPU usage and break IO optimisations is not the way to address a list length that has apparently excceeded the bounds built into the code... -Dave. -- Dave Chinner dgc@kernel.org