netdev - [PATCH net-next] net: avoid irqsave in skb_defer_free

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Date:   Tue, 17 Jan 2023 13:29:07 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     netdev@...r.kernel.org
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>, edumazet@...gle.com,
        pabeni@...hat.com
Subject: [PATCH net-next] net: avoid irqsave in skb_defer_free_flush

The spin_lock irqsave/restore API variant in skb_defer_free_flush can
be replaced with the faster spin_lock irq variant, which doesn't need
to read and restore the CPU flags.

Using the unconditional irq "disable/enable" API variant is safe,
because the skb_defer_free_flush() function is only called during
NAPI-RX processing in net_rx_action(), where it is known the IRQs
are enabled.

Expected gain is 14 cycles from avoiding reading and restoring CPU
flags in a spin_lock_irqsave/restore operation, measured via a
microbencmark kernel module[1] on CPU E5-1650 v4 @ 3.60GHz.

Microbenchmark overhead of spin_lock+unlock:
 - spin_lock_unlock_irq     cost: 34 cycles(tsc)  9.486 ns
 - spin_lock_unlock_irqsave cost: 48 cycles(tsc) 13.567 ns

We don't expect to see a measurable packet performance gain, as
skb_defer_free_flush() is called infrequently once per NIC device NAPI
bulk cycle and conditionally only if SKBs have been deferred by other
CPUs via skb_attempt_defer_free().

[1] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/time_bench_sample.c

Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
---
 net/core/dev.c |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index cf78f35bc0b9..9c60190fe352 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6616,17 +6616,16 @@ static int napi_threaded_poll(void *data)
 static void skb_defer_free_flush(struct softnet_data *sd)
 {
 	struct sk_buff *skb, *next;
-	unsigned long flags;

 	/* Paired with WRITE_ONCE() in skb_attempt_defer_free() */
 	if (!READ_ONCE(sd->defer_list))
 		return;

-	spin_lock_irqsave(&sd->defer_lock, flags);
+	spin_lock_irq(&sd->defer_lock);
 	skb = sd->defer_list;
 	sd->defer_list = NULL;
 	sd->defer_count = 0;
-	spin_unlock_irqrestore(&sd->defer_lock, flags);
+	spin_unlock_irq(&sd->defer_lock);

 	while (skb != NULL) {
 		next = skb->next;