Re: [PATCH] percpu_counter: reduce i-cache footprint of percpu_counter_add_batch() fast path

[Mateusz Guzik <mjguzik@gmail.com>]

So I just verified gcc 15 has this problem fixed. So one can argue the
issue will go away on its own given enough time, hence the patch can
be dropped if there are any concerns.

On Wed, Nov 19, 2025 at 10:08 PM Mateusz Guzik <mjguzik@gmail.com> wrote:
>
> When compiled with gcc 14.2 for the x86-64 architecture with ORC frame
> unwinder the fast path still has the most unfortunate size of 66 bytes,
> in part from register spilling to falicitate the fallback.
>
> Moving it out solves the problem by keeping it just below 64 bytes.
>
> Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
> ---
>  lib/percpu_counter.c | 30 ++++++++++++++++++++----------
>  1 file changed, 20 insertions(+), 10 deletions(-)
>
> diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
> index 2891f94a11c6..0cf6f1101903 100644
> --- a/lib/percpu_counter.c
> +++ b/lib/percpu_counter.c
> @@ -89,24 +89,34 @@ EXPORT_SYMBOL(percpu_counter_set);
>   * Safety against interrupts is achieved in 2 ways:
>   * 1. the fast path uses local cmpxchg (note: no lock prefix)
>   * 2. the slow path operates with interrupts disabled
> + *
> + * Slowpath is implemented as a separate routine to reduce register spillage by gcc.
>   */
> -void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch)
> +static void noinline percpu_counter_add_batch_slowpath(struct percpu_counter *fbc,
> +                                                      s64 amount, s32 batch)
>  {
>         s64 count;
>         unsigned long flags;
>
> +       raw_spin_lock_irqsave(&fbc->lock, flags);
> +       /*
> +        * Note: by now we might have migrated to another CPU or the value
> +        * might have changed.
> +        */
> +       count = __this_cpu_read(*fbc->counters);
> +       fbc->count += count + amount;
> +       __this_cpu_sub(*fbc->counters, count);
> +       raw_spin_unlock_irqrestore(&fbc->lock, flags);
> +}
> +
> +void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch)
> +{
> +       s64 count;
> +
>         count = this_cpu_read(*fbc->counters);
>         do {
>                 if (unlikely(abs(count + amount) >= batch)) {
> -                       raw_spin_lock_irqsave(&fbc->lock, flags);
> -                       /*
> -                        * Note: by now we might have migrated to another CPU
> -                        * or the value might have changed.
> -                        */
> -                       count = __this_cpu_read(*fbc->counters);
> -                       fbc->count += count + amount;
> -                       __this_cpu_sub(*fbc->counters, count);
> -                       raw_spin_unlock_irqrestore(&fbc->lock, flags);
> +                       percpu_counter_add_batch_slowpath(fbc, amount, batch);
>                         return;
>                 }
>         } while (!this_cpu_try_cmpxchg(*fbc->counters, &count, count + amount));
> --
> 2.48.1
>