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

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Message-ID: <878qv90x6w.ffs@tglx>
Date: Mon, 30 Sep 2024 21:16:07 +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 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.
>> 
>>      old = mg_floor
>>                                 mono = T1;
>>                                 mg_floor = mono
>> preemption
>> 
>>      do {
>>         mono = T2;
>>      }
>> 
>>      cmpxchg fails and the function returns a value based on T1
>> 
>> No?
>> 
>> 
>
> 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.
>
> 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.

That really needs to be documented. A similar scenario exists
vs. ktime_get_coarse_real_ts64_mg().

Thanks,

        tglx