linux-kernel - Re: [PATCH v2 12/25] timekeeping: Add struct tk_data as argument to timekeeping

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Message-ID: <CANDhNCrShVYQ4OXPVq9MAFGZhGqZ1-yEPpe_aCTpWCBRLE2XTA@mail.gmail.com>
Date: Thu, 24 Oct 2024 14:29:38 -0700
From: John Stultz <jstultz@...gle.com>
To: Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc: Frederic Weisbecker <frederic@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, 
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
	Miroslav Lichvar <mlichvar@...hat.com>, Richard Cochran <richardcochran@...il.com>, 
	Christopher S Hall <christopher.s.hall@...el.com>
Subject: Re: [PATCH v2 12/25] timekeeping: Add struct tk_data as argument to timekeeping_update()

On Wed, Oct 9, 2024 at 1:29 AM Anna-Maria Behnsen
<anna-maria@...utronix.de> wrote:
>
> From: Anna-Maria Behnsen <anna-maria@...utronix.de>
>
> Updates of the timekeeper are done in two ways:
>
>  1. Updating timekeeper and afterwards memcpy()'ing the result into
>     shadow_timekeeper using timekeeping_update(). Used everywhere for
>     updates except in timekeeping_advance(); the sequence counter protected
>     region starts before the first change to the timekeeper is done.
>
>  2. Updating shadow_timekeeper and then memcpy()'ing the result into
>     timekeeper.  Used only by in timekeeping_advance(); The seqence counter
>     protected region is only around timekeeping_update() and the memcpy for
>     copy from shadow to timekeeper.
>
> The second option is fast path optimized. The sequence counter protected
> region is as short as possible.
>
> As this behaviour is mainly documented by commit messages, but not in code,
> it makes the not easy timekeeping code more complicated to read.
>
> There is no reason why updates to the timekeeper can't use the optimized
> version everywhere. With this, the code will be cleaner, as code is reused
> instead of duplicated.
>
> To be able to access tk_data which contains all required information, add a
> pointer to tk_data as an argument to timekeeping_update(). With that
> convert the comment about holding the lock into a lockdep assert.
>
> No functional change.
>
> Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>

Acked-by: John Stultz <jstultz@...gle.com>