linux-kernel - Re: [PATCH v2 13/25] timekeeping: Split out timekeeper update of timekeeping

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Message-ID: <CANDhNCrTzUODaXyVdX2t+d+0s3cxWi4KV4MxJNjJrGh+0WxEEA@mail.gmail.com>
Date: Thu, 24 Oct 2024 14:53:50 -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 13/25] timekeeping: Split out timekeeper update of timekeeping_advanced()

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>
>
> timekeeping_advance() is the only optimized function which uses
> shadow_timekeeper for updating the real timekeeper to keep the sequence
> counter protected region as small as possible.
>
> To be able to transform timekeeper updates in other functions to use the
> same logic, split out functionality into a separate function
> timekeeper_update_staged().
>
> While at it, document the reason why the sequence counter must be write
> held over the call to timekeeping_update() and the copying to the real
> timekeeper and why using a pointer based update is suboptimal.
>
> No functional change.
>
> Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
> ---
>  kernel/time/timekeeping.c | 43 +++++++++++++++++++++++++++----------------
>  1 file changed, 27 insertions(+), 16 deletions(-)
>
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 878f9606946d..fcb2b8b232d2 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -780,7 +780,32 @@ static void timekeeping_update(struct tk_data *tkd, struct timekeeper *tk, unsig
>          * timekeeper structure on the next update with stale data
>          */
>         if (action & TK_MIRROR)
> -               memcpy(&tk_core.shadow_timekeeper, &tk_core.timekeeper, sizeof(tk_core.timekeeper));
> +               memcpy(&tkd->shadow_timekeeper, tk, sizeof(*tk));
> +}
> +
> +static void timekeeping_update_staged(struct tk_data *tkd, unsigned int action)

Minor nit I realized as I saw how this was used later on:
timekeeping_update_staged() isn't super clear right off. Maybe
timekeeping_update_from_shadow() might make it more clear?

thanks
-john