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

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Message-ID: <8734lgyote.ffs@tglx>
Date: Tue, 01 Oct 2024 14:45:17 +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 Tue, Oct 01 2024 at 05:45, Jeff Layton wrote:
> On Mon, 2024-09-30 at 23:35 +0200, Thomas Gleixner wrote:
>> > I certainly wouldn't rule out a workqueue job calling that function,
>> > but this is something we do while dirtying an inode, and that's not
>> > typically done in interrupt context.
>> 
>> The reason I'm asking is that if it's always syscall context,
>> i.e. write() or io_uring()/RPC request etc., then you can avoid the
>> whole global floor value dance and make it strictly per thread, which
>> simplifies the exercise significantly.
>> 
>
> I'm not sure I follow what you're proposing here.
>
> Consider two threads doing writes serially to different files. IOW, the
> second thread enters the write() syscall after the first thread returns
> from its write(). In that situation, the second timestamp mustn't
> appear to be earlier than the first (assuming no backward clock jump,
> of course).
>
> How would we ensure that with only per-thread structures?

Bah. Right. Ignore my sleep deprived rambling.