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Message-ID: <874j5x0vwq.ffs@tglx> Date: Mon, 30 Sep 2024 21:43:49 +0200 From: Thomas Gleixner <tglx@...utronix.de> To: Jeff Layton <jlayton@...nel.org>, John Stultz <jstultz@...gle.com>, Stephen Boyd <sboyd@...nel.org>, Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Jonathan Corbet <corbet@....net>, Chandan Babu R <chandan.babu@...cle.com>, "Darrick J. Wong" <djwong@...nel.org>, Theodore Ts'o <tytso@....edu>, Andreas Dilger <adilger.kernel@...ger.ca>, Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>, David Sterba <dsterba@...e.com>, Hugh Dickins <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, Chuck Lever <chuck.lever@...cle.com>, Vadim Fedorenko <vadim.fedorenko@...ux.dev> Cc: Randy Dunlap <rdunlap@...radead.org>, linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, linux-doc@...r.kernel.org, linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org, linux-btrfs@...r.kernel.org, linux-nfs@...r.kernel.org, linux-mm@...ck.org Subject: Re: [PATCH v8 01/11] timekeeping: move multigrain timestamp floor handling into timekeeper 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. > > 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. > > 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. The comments I made about races and the clock_settime() inconsistency vs. the change log aside, I don't see room for improvement there. 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. Thanks, tglx
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