Re: [PATCH v2 18/25] timekeeping: Rework change_clocksource() to use shadow_timekeeper

[John Stultz]

On Wed, Oct 9, 2024 at 1:29 AM Anna-Maria Behnsen
<anna-maria@xxxxxxxxxxxxx> wrote:
>
> From: Anna-Maria Behnsen <anna-maria@xxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxx>
> ---
>  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@xxxxxxxxxx>