Re: [PATCH net-next v3 01/18] net: page_pool: Don't recycle into cache on PREEMPT_RT

[Ilias Apalodimas <ilias.apalodimas@linaro.org>]

Hi Sebastian

On Wed, 30 Apr 2025 at 15:48, Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
>
> With preemptible softirq and no per-CPU locking in local_bh_disable() on
> PREEMPT_RT the consumer can be preempted while a skb is returned.
>
> Avoid the race by disabling the recycle into the cache on PREEMPT_RT.

I am not expert on PREEMPT_RT, but this sounds reasonable.
Did you have time to test this at all? There's a kernel module Jesper
originally authored to track regressions, which unfortunately isn't
upstreamed yet [0].
Any chance you can quickly spin it to get some numbers?

[0] https://lore.kernel.org/netdev/20250309084118.3080950-1-almasrymina@google.com/

Cheers
/Ilias
>
> Cc: Jesper Dangaard Brouer <hawk@kernel.org>
> Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  net/core/page_pool.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 7745ad924ae2d..ba8803c2c0b20 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -805,6 +805,10 @@ static bool page_pool_napi_local(const struct page_pool *pool)
>         const struct napi_struct *napi;
>         u32 cpuid;
>
> +       /* On PREEMPT_RT the softirq can be preempted by the consumer */
> +       if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +               return false;
> +
>         if (unlikely(!in_softirq()))
>                 return false;
>
> --
> 2.49.0
>