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Message-ID: <167421646327.1321776.7390743166998776914.stgit@firesoul>
Date: Fri, 20 Jan 2023 13:07:43 +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, jacob.e.keller@...el.com
Subject: [PATCH net-next V2] 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
Reviewed-by: Jacob Keller <jacob.e.keller@...el.com>
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;
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