Commit 9c94ae6b authored Sep 28, 2025 by Eric Dumazet Committed by Paolo Abeni Sep 30, 2025

net: make softnet_data.defer_count an atomic



This is preparation work to remove the softnet_data.defer_lock,
as it is contended on hosts with large number of cores.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250928084934.3266948-2-edumazet@google.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>

parent 2c0592bd

include/linux/netdevice.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -3538,7 +3538,7 @@ struct softnet_data {

		/* Another possibly contended cache line */
		spinlock_t defer_lock ____cacheline_aligned_in_smp;
		int defer_count;
		atomic_t defer_count;
		int defer_ipi_scheduled;
		struct sk_buff *defer_list;
		call_single_data_t defer_csd;

net/core/dev.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -6726,7 +6726,7 @@ static void skb_defer_free_flush(struct softnet_data *sd)
		spin_lock(&sd->defer_lock);
		skb = sd->defer_list;
		sd->defer_list = NULL;
		sd->defer_count = 0;
		atomic_set(&sd->defer_count, 0);
		spin_unlock(&sd->defer_lock);

		while (skb != NULL) {

net/core/skbuff.c

+2 −4

Original line number	Diff line number	Diff line
		@@ -7202,14 +7202,12 @@ nodefer: kfree_skb_napi_cache(skb);

		sd = &per_cpu(softnet_data, cpu);
		defer_max = READ_ONCE(net_hotdata.sysctl_skb_defer_max);
		if (READ_ONCE(sd->defer_count) >= defer_max)
		if (atomic_read(&sd->defer_count) >= defer_max)
		goto nodefer;

		spin_lock_bh(&sd->defer_lock);
		/* Send an IPI every time queue reaches half capacity. */
		kick = sd->defer_count == (defer_max >> 1);
		/* Paired with the READ_ONCE() few lines above */
		WRITE_ONCE(sd->defer_count, sd->defer_count + 1);
		kick = (atomic_inc_return(&sd->defer_count) - 1) == (defer_max >> 1);

		skb->next = sd->defer_list;
		/* Paired with READ_ONCE() in skb_defer_free_flush() */