linux-kernel - Re: [PATCH v8 01/11] timekeeping: move multigrain timestamp floor handling into timekeeper

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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