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

Dragos Tatulea
2026-04-28T17:52:19Z

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.

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.

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);
+
+ /* Timer started, so need for another call to
+ * napi->poll().
*/
- timeout = napi_get_gro_flush_timeout(napi);
+ goto out;
}
}

@@ -6938,8 +6933,12 @@ static void busy_poll_stop(struct napi_struct *napi, void *have_poll_lock,
*/
trace_napi_poll(napi, rc, budget);
netpoll_poll_unlock(have_poll_lock);
- if (rc == budget)
- __busy_poll_stop(napi, timeout);
+ if (rc == budget) {
+ gro_normal_list(&napi->gro);
+ __napi_schedule(napi);
+ }
+
+out:
bpf_net_ctx_clear(bpf_net_ctx);
local_bh_enable();
}
--
2.43.0