From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 85755CDD1D5 for ; Mon, 30 Sep 2024 20:12:19 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 05587280025; Mon, 30 Sep 2024 16:12:19 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 006A26B0125; Mon, 30 Sep 2024 16:12:18 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id D69E7280025; Mon, 30 Sep 2024 16:12:18 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id B619F6B0123 for ; Mon, 30 Sep 2024 16:12:18 -0400 (EDT) Received: from smtpin11.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay10.hostedemail.com (Postfix) with ESMTP id 667D7C0860 for ; Mon, 30 Sep 2024 20:12:18 +0000 (UTC) X-FDA: 82622501556.11.31D6D8D Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by imf01.hostedemail.com (Postfix) with ESMTP id A3AE94001E for ; Mon, 30 Sep 2024 20:12:16 +0000 (UTC) Authentication-Results: imf01.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=SRnDlNtQ; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf01.hostedemail.com: domain of jlayton@kernel.org designates 139.178.84.217 as permitted sender) smtp.mailfrom=jlayton@kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1727726997; h=from:from:sender: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:dkim-signature; bh=ri3ziguyp08Wemifs+asP9srk4bMRRbJTCmjZtS52QQ=; b=2z/eIcrH4nsviDS8bsf1Y4vmC+dv6uOv5u1ojFGz7O3GgcSEqQgYdo57RqXaMQCvhKoWN8 scMPLXbVQs7xdgdsLCBm7n+2XXUp+J77F2ZNknEgRDGVdXGoVZry9ZCc22uUe63gHGEyx+ G3GA9f4TcSGTYSnWRdc8qb4aOBkoayc= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1727726997; a=rsa-sha256; cv=none; b=tUwTNd35sGBCp5D9ufLNqQeUT54ohZo7C7SKy9CZFdb+kryxHAdsTAVbxcvnpYZG2hMtQR QqTyq4cbfCVxnoszJiAPJo6vQdPVzoUskpdts+887jT0M8t5DDW1Z77a2npp1NXYpmYqTS F1OpTT9d0g0owzKpPJXd6B2fXrXytGs= ARC-Authentication-Results: i=1; imf01.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=SRnDlNtQ; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf01.hostedemail.com: domain of jlayton@kernel.org designates 139.178.84.217 as permitted sender) smtp.mailfrom=jlayton@kernel.org Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by dfw.source.kernel.org (Postfix) with ESMTP id 8A9095C0603; Mon, 30 Sep 2024 20:12:11 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id B60A7C4CEC7; Mon, 30 Sep 2024 20:12:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1727727135; bh=ri3ziguyp08Wemifs+asP9srk4bMRRbJTCmjZtS52QQ=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=SRnDlNtQ5OhMgJwg5IawR52VZ/F8u6p0IT5cDlBzfz1XsRc6H7lVcwkdjum/MEUkl 78stgbMwqTThRp2lXPtxwtI+vGWEbLYjLaAoSgyldhXm2SfE2PnUhYIXxIx7UCjirC ZyugAYCXhyDNqJUOh42gMV5mKI+HC71XOOc6KsHrZ9w794LRZfUHtYDtJ+IDtCZYe+ Uq2xEZZeuDfE1AgSS2xe5t4Uqj0k+8Gwn8Sb/n1abt8q7ZO+Xgu4ayM4QqAMidP93G 3ypgscLJ3vXZiNnGE/PSCNSI8i5l/pyLTQaDyiVVM3SrUSy2VsW1f4gH5ZdNF9JwYq tC1EOSSAAIF1g== Message-ID: Subject: Re: [PATCH v8 01/11] timekeeping: move multigrain timestamp floor handling into timekeeper From: Jeff Layton To: Thomas Gleixner , John Stultz , Stephen Boyd , Alexander Viro , Christian Brauner , Jan Kara , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Jonathan Corbet , Chandan Babu R , "Darrick J. Wong" , Theodore Ts'o , Andreas Dilger , Chris Mason , Josef Bacik , David Sterba , Hugh Dickins , Andrew Morton , Chuck Lever , Vadim Fedorenko Cc: Randy Dunlap , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org Date: Mon, 30 Sep 2024 16:12:11 -0400 In-Reply-To: <874j5x0vwq.ffs@tglx> References: <20240914-mgtime-v8-0-5bd872330bed@kernel.org> <20240914-mgtime-v8-1-5bd872330bed@kernel.org> <87a5g79aag.ffs@tglx> <874j5x0vwq.ffs@tglx> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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X-Rspam-User: X-HE-Tag: 1727727136-890001 X-HE-Meta: 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 OizQStqC jClbRzP7YQ/lsvjdMD6nN9jjpIkZVoQPOWZGT5MC9yj+xda+ib2AunLxHky/2/4PU69QhZZEZtfisUAsAzTjm1hGjeGuj/7oY2LUNLGiJCCIHdvx6bp0ShDciURyT6EMKEaatKlx+cO9/Bb0jm0jTebH+CCM9ccDvpsnMky3iHWZvGysW6QrvT1kjrgoalDDt+oBk4MHoa5D36oQcmUxCyBje53k+RnEnUhcMrAOBI/gsaPZ+9Ya2MuMH8A== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, 2024-09-30 at 21:43 +0200, Thomas Gleixner wrote: > On Thu, Sep 19 2024 at 18:50, Jeff Layton wrote: > > The fix for this is to establish a floor value for the coarse-grained > > clock. When stamping a file with a fine-grained timestamp, we update > > the floor value with the current monotonic time (using cmpxchg). Then > > later, when a coarse-grained timestamp is requested, check whether the > > floor is later than the current coarse-grained time. If it is, then the > > kernel will return the floor value (converted to realtime) instead of > > the current coarse-grained clock. That allows us to maintain the > > ordering guarantees. > >=20 > > My original implementation of this tracked the floor value in > > fs/inode.c (also using cmpxchg), but that caused a performance > > regression, mostly due to multiple calls into the timekeeper functions > > with seqcount loops. By adding the floor to the timekeeper we can get > > that back down to 1 seqcount loop. > >=20 > > Let me know if you have more questions about this, or suggestions about > > how to do this better. The timekeeping code is not my area of expertise > > (obviously) so I'm open to doing this a better way if there is one. >=20 > The comments I made about races and the clock_settime() inconsistency > vs. the change log aside, I don't see room for improvement there. >=20 > What worries me is the atomic_cmpxchg() under contention on large > machines, but as it is not a cmpxchg() loop it might be not completely > horrible. >=20 Thanks for taking a look. The code does try to avoid requesting a fine-grained timestamp unless there is no other option. In practice, that means that a write()+stat()+write() has to occur within the same coarse-grained timer tick. That obviously does happen, but I'm hoping it's not very frequent on real-world workloads. The main place where we see a performance hit from this set is the will-it-scale pipe test, which does 1-byte writes and reads on a pipe as fast as possible. The previous patchset tracked the floor inside fs/inode.c, and each timestamp update on the pipe inode could make multiple trips into the timekeeper seqcount loops. That added extra barriers and slowed things down. This patchset moves the code into the timekeeper, which means only a single barrier. That gets the cost down, but it's not entirely free. There is some small cost to dealing with the floor, regardless. --=20 Jeff Layton