Re: [RFC PATCH net-next 2/2] net: napi: Skip poll when arming GRO timer in busy poll

Björn Töpel
2026-04-29T12:37:17Z

Dragos Tatulea <dtatulea@nvidia.com> writes:

> From: Martin Karsten <mkarsten@uwaterloo.ca>
>
> As referenced in the previous patch, having the GRO timer scheduled
> while poll is running can lead to issues.

Outline the issues here as well, so we don't need to cross-reference the
kernel logs.

> Skip the extra call to napi->poll() when the GRO timer is armed. This
> removes the need for having a separate __busy_poll_stop routine and its
> code is moved directly into the relevant places in busy_poll_stop.

Nice!

> Signed-off-by: Martin Karsten <mkarsten@uwaterloo.ca>
> Co-developed-by: Dragos Tatulea <dtatulea@nvidia.com>
> Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
> ---
> net/core/dev.c | 61 +++++++++++++++++++++++++-------------------------
> 1 file changed, 30 insertions(+), 31 deletions(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 1487d4946dcf..d4829c2484f5 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -6869,26 +6869,6 @@ static void skb_defer_free_flush(void)
>
> #if defined(CONFIG_NET_RX_BUSY_POLL)
>
> -static void __busy_poll_stop(struct napi_struct *napi, unsigned long timeout)
> -{
> - unsigned long flags;
> -
> - if (!timeout) {
> - gro_normal_list(&napi->gro);
> - __napi_schedule(napi);
> - return;
> - }
> -
> - /* Flush too old packets. If HZ < 1000, flush all packets */
> - gro_flush_normal(&napi->gro, HZ >= 1000);
> -
> - local_irq_save(flags);
> - hrtimer_start(&napi->timer, ns_to_ktime(timeout),
> - HRTIMER_MODE_REL_PINNED);
> - clear_bit(NAPI_STATE_SCHED, &napi->state);
> - local_irq_restore(flags);
> -}
> -
> enum {
> NAPI_F_PREFER_BUSY_POLL = 1,
> NAPI_F_END_ON_RESCHED = 2,
> @@ -6898,14 +6878,14 @@ static void busy_poll_stop(struct napi_struct *napi, void *have_poll_lock,
> unsigned flags, u16 budget)
> {
> struct bpf_net_context __bpf_net_ctx, *bpf_net_ctx;
> - unsigned long timeout = 0;
> + unsigned long timeout;
> int rc;
>
> /* Busy polling means there is a high chance device driver hard irq
> * could not grab NAPI_STATE_SCHED, and that NAPI_STATE_MISSED was
> * set in napi_schedule_prep().
> - * Since we are about to call napi->poll() once more, we can safely
> - * clear NAPI_STATE_MISSED.
> + * Since we either call napi->poll() once more or start the timer,
> + * we can safely clear NAPI_STATE_MISSED.
> *
> * Note: x86 could use a single "lock and ..." instruction
> * to perform these two clear_bit()
> @@ -6918,13 +6898,28 @@ static void busy_poll_stop(struct napi_struct *napi, void *have_poll_lock,
>
> if (flags & NAPI_F_PREFER_BUSY_POLL) {
> napi->defer_hard_irqs_count = napi_get_defer_hard_irqs(napi);
> - if (napi->defer_hard_irqs_count) {
> - /* Timer will be scheduled after napi poll to avoid
> - * firing during a slow poll which could cause the
> - * queue to get stuck with interrupts disabled and no
> - * scheduled timer.
> + timeout = napi_get_gro_flush_timeout(napi);
> + if (napi->defer_hard_irqs_count && timeout) {
> + unsigned long flags;
> +
> + /* Drop prefer-busy state as in napi_complete_done(). */
> + clear_bit(NAPI_STATE_PREFER_BUSY_POLL, &napi->state);
> + netpoll_poll_unlock(have_poll_lock);
> +
> + /* Flush too old packets. If HZ < 1000, flush all
> + * packets.
> + */
> + gro_flush_normal(&napi->gro, HZ >= 1000);
> + local_irq_save(flags);
> + hrtimer_start(&napi->timer, ns_to_ktime(timeout),
> + HRTIMER_MODE_REL_PINNED);
> + clear_bit(NAPI_STATE_SCHED, &napi->state);
> + local_irq_restore(flags);

(Same swap arm timer/clear as patch 1.)

> +
> + /* Timer started, so need for another call to
^
"so *NO* need for another..."

Björn