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 A36A114F9D9; Mon, 30 Sep 2024 19:37:18 +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=1727725038; cv=none; b=V8wCYg6JiJcSP7ikjc8Iua/9Tp7YmM3HY0NKR/8x79VIHdBBFr7DKEMIjoIgEw8wyqu0MsjvUCFAXmb0HHjKtB07ZrrMc9+8VGpz/YRExHSnUppibGB6S17vPjmWBnb4w4iR2ZdWM5k4LvtCeJPBdKZptv0StkwTUgbkJ2o3++8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727725038; c=relaxed/simple; bh=AUTU96iyYXkfHgK0dTxI/fkVIV97CDq1YiK27OAFx6Y=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=qGI+3Wtw6fDj8nlu1UvALXqZJpVTmC0y05Bt6Jdh2zP1bpmhTrf0w8OufmEesba4ZOTx/P+uzh3bkEbZBTpLR31HeZv2g/MViF60YlfdrZ70v5ltKbSHzKdMvJ3qp3W7MkaaRYIxSgqmYBF1tzcsJwzVnQPGxXhzk187LLwW8sI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=kipEP3JE; 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="kipEP3JE" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AEB25C4CEC7; Mon, 30 Sep 2024 19:37:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1727725038; bh=AUTU96iyYXkfHgK0dTxI/fkVIV97CDq1YiK27OAFx6Y=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=kipEP3JEc3TcWuF4vydlPebTPQQ04LTIeRGzHif9UywdJDOTEIjWFIlbPXaCE6M71 YDl/98QtodAXktgxMmV6vskAmIYSVofi8HDWkQtUQrV0QaI81TsTdGYASLh8TusI4z vUFCQF65pw1pIBb9RQA5pE++2wuls/BcVdVfgN3wz5DDPVuzgxepELmcLk8zKL4DtG FF5nUn+YK7yPzk0HQfM3L23JdXsJ54XMRHFop0DT1Mlvv0wWEi3vXWnUJEyRgDa4xo Kwzho+cGca4raHNlB0IGaXX9PYVpxKBAMMTVshtIn0reot+LkHvtZlJuKjPZ3Fn8X3 hBNPmdfQVj5Qg== Message-ID: <4933075b1023f466edb516e86608e0938de28c1d.camel@kernel.org> 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 15:37:14 -0400 In-Reply-To: <878qv90x6w.ffs@tglx> References: <20240914-mgtime-v8-0-5bd872330bed@kernel.org> <20240914-mgtime-v8-1-5bd872330bed@kernel.org> <87a5g79aag.ffs@tglx> <874j6f99dg.ffs@tglx> <878qv90x6w.ffs@tglx> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Mon, 2024-09-30 at 21:16 +0200, Thomas Gleixner wrote: > On Mon, Sep 16 2024 at 06:57, Jeff Layton wrote: > > On Mon, 2024-09-16 at 12:32 +0200, Thomas Gleixner wrote: > > > > 'Something has changed' is a truly understandable technical > > > > explanation. > > >=20 > > > old =3D mg_floor > > > mono =3D T1; > > > mg_floor =3D mono > > > preemption > > >=20 > > > do { > > > mono =3D T2; > > > } > > >=20 > > > cmpxchg fails and the function returns a value based on T1 > > >=20 > > > No? > > >=20 > > >=20 > >=20 > > Packing for LPC, so I can't respond to all of these just now, but I > > will later. You're correct, but either outcome is OK. > >=20 > > The requirement is that we don't hand out any values that were below > > the floor at the time that the task entered the kernel. Since the time > > changed while the task was already inside the kernel, either T1 or T2 > > would be valid timestamps. >=20 > That really needs to be documented. A similar scenario exists > vs. ktime_get_coarse_real_ts64_mg(). >=20 Yes. I have the following section in the multigrain-ts.rst file that gets added in patch 7 of this series. I'll also plan to add some extra wording about how backward realtime clock jumps can affect ordering: Inode Timestamp Ordering =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D In addition to providing info about changes to individual files, file = =20 timestamps also serve an important purpose in applications like "make". The= se =20 programs measure timestamps in order to determine whether source files migh= t be =20 newer than cached objects. = =20 Userland applications like make can only determine ordering based on = =20 operational boundaries. For a syscall those are the syscall entry and exit = =20 points. For io_uring or nfsd operations, that's the request submission and = =20 response. In the case of concurrent operations, userland can make no = =20 determination about the order in which things will occur. For instance, if a single thread modifies one file, and then another file i= n =20 sequence, the second file must show an equal or later mtime than the first.= The =20 same is true if two threads are issuing similar operations that do not over= lap =20 in time. If however, two threads have racing syscalls that overlap in time, then the= re =20 is no such guarantee, and the second file may appear to have been modified = =20 before, after or at the same time as the first, regardless of which one was= =20 submitted first. --=20 Jeff Layton