diff mbox series

[net-next,V2] net: avoid irqsave in skb_defer_free_flush

Message ID 167421646327.1321776.7390743166998776914.stgit@firesoul (mailing list archive)
State Accepted
Commit 3176eb82681ec9c8af31c6588ddedcc6cfb9e445
Delegated to: Netdev Maintainers
Headers show
Series [net-next,V2] net: avoid irqsave in skb_defer_free_flush | expand

Checks

Context Check Description
netdev/tree_selection success Clearly marked for net-next
netdev/fixes_present success Fixes tag not required for -next series
netdev/subject_prefix success Link
netdev/cover_letter success Single patches do not need cover letters
netdev/patch_count success Link
netdev/header_inline success No static functions without inline keyword in header files
netdev/build_32bit success Errors and warnings before: 8 this patch: 8
netdev/cc_maintainers warning 1 maintainers not CCed: petrm@nvidia.com
netdev/build_clang success Errors and warnings before: 1 this patch: 1
netdev/module_param success Was 0 now: 0
netdev/verify_signedoff success Signed-off-by tag matches author and committer
netdev/check_selftest success No net selftest shell script
netdev/verify_fixes success No Fixes tag
netdev/build_allmodconfig_warn success Errors and warnings before: 8 this patch: 8
netdev/checkpatch success total: 0 errors, 0 warnings, 0 checks, 19 lines checked
netdev/kdoc success Errors and warnings before: 0 this patch: 0
netdev/source_inline success Was 0 now: 0

Commit Message

Jesper Dangaard Brouer Jan. 20, 2023, 12:07 p.m. UTC 
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@intel.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 net/core/dev.c |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

Comments

patchwork-bot+netdevbpf@kernel.org Jan. 24, 2023, 6:10 a.m. UTC | #1 
Hello:

This patch was applied to netdev/net-next.git (master)
by Jakub Kicinski <kuba@kernel.org>:

On Fri, 20 Jan 2023 13:07:43 +0100 you wrote:
> 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.
> 
> [...]

Here is the summary with links:
  - [net-next,V2] net: avoid irqsave in skb_defer_free_flush
    https://git.kernel.org/netdev/net-next/c/3176eb82681e

You are awesome, thank you!
diff mbox series

Patch

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;