linux-kernel - Re: [PATCH v2 18/25] timekeeping: Rework change_clocksource() to use shadow

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Message-ID: <CANDhNCrvQeEzDuY+K+fV73tNzOjacsu9JY3rjXqKfAgfG8rgyg@mail.gmail.com>
Date: Thu, 24 Oct 2024 14:58:54 -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 18/25] timekeeping: Rework change_clocksource() to use shadow_timekeeper

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 can be done by operating on the shadow timekeeper
> and afterwards copying the result into the real timekeeper. This has the
> advantage, that the sequence count write protected region is kept as small
> as possible.
>
> Convert change_clocksource() to use this scheme.
>
> Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
> ---
>  kernel/time/timekeeping.c | 18 +++++++-----------
>  1 file changed, 7 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 051041e92e54..6d21e9bf5c35 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -1596,9 +1596,7 @@ static void __timekeeping_set_tai_offset(struct timekeeper *tk, s32 tai_offset)
>   */
>  static int change_clocksource(void *data)
>  {
> -       struct timekeeper *tk = &tk_core.timekeeper;
>         struct clocksource *new = data, *old = NULL;
> -       unsigned long flags;
>
>         /*
>          * If the clocksource is in a module, get a module reference.
> @@ -1614,16 +1612,14 @@ static int change_clocksource(void *data)
>                 return 0;
>         }
>
> -       raw_spin_lock_irqsave(&tk_core.lock, flags);
> -       write_seqcount_begin(&tk_core.seq);
> -
> -       timekeeping_forward_now(tk);
> -       old = tk->tkr_mono.clock;
> -       tk_setup_internals(tk, new);
> -       timekeeping_update(&tk_core, tk, TK_UPDATE_ALL | TK_MIRROR);
> +       scoped_guard (raw_spinlock_irqsave, &tk_core.lock) {
> +               struct timekeeper *tk = &tk_core.shadow_timekeeper;

Same nit: tk_shadow maybe?

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