On 05/07/2015 04:41 AM, Denys Vlasenko wrote:
These functions compile to ~60 bytes of machine code each.
With this .config: http://busybox.net/~vda/kernel_config
there are 617 calls to netif_tx_stop_queue()
and 49 calls to netif_tx_stop_all_queues() in vmlinux.
Code size is reduced by 27 kbytes:
text data bss dec hex filename
82426986 22255416 20627456 125309858 77813a2 vmlinux.before
82399481 22255416 20627456 125282353 777a831 vmlinux
It may seem strange that a seemingly simple code like one in
netif_tx_stop_queue() compiles to ~60 bytes of code.
Well, it's true. Here's its disassembly:
netif_tx_stop_queue:
e8 b0 15 4d 00 callq <__fentry__>
This bit was added because you converted this to a function.
48 85 ff test %rdi,%rdi
75 25 jne <netif_tx_stop_queue+0x2f>
This bit is your WARN_ON test
55 push %rbp
be 7a 18 00 00 mov $0x187a,%esi
48 c7 c7 50 59 d8 85 mov $.rodata+0x1d85950,%rdi
48 89 e5 mov %rsp,%rbp
e8 54 5a 7d fd callq <warn_slowpath_null>
48 c7 c7 5f 59 d8 85 mov $.rodata+0x1d8595f,%rdi
31 c0 xor %eax,%eax
e8 b0 47 48 00 callq <printk>
eb 09 jmp <netif_tx_stop_queue+0x38>
This is the WARN_ON action. One thing you might try doing is moving
this to a function of its own instead of moving the entire thing out of
being an inline. You may find you still get most of the space savings
as I wonder if the string for the printk isn't being duplicated for each
caller.
f0 80 8f e0 01 00 00 01 lock orb $0x1,0x1e0(%rdi)
This is your set bit operation. If you were to drop the whole WARN_ON
then this is the only thing you would be inlining. That is only 8 bytes
in size which would probably be comparable to the callq and register
sorting needed for a function call.
c3 retq
5d pop %rbp
c3 retq
The rest of this is just more function overhead, one return for your
standard path, and a pop and a return for the WARN_ON path.
This causes gcc to auto-deinline it before this patch, but with 203 separate
copies in each module which uses this function:
$ nm --size-sort vmlinux.before | grep -e ' netif_tx_stop_queue$' | wc -l
203
Signed-off-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
CC: David S. Miller <davem@xxxxxxxxxxxxx>
CC: Jiri Pirko <jpirko@xxxxxxxxxx>
CC: linux-kernel@xxxxxxxxxxxxxxx
CC: netdev@xxxxxxxxxxxxxxx
CC: netfilter-devel@xxxxxxxxxxxxxxx
---
Have you done any performance testing on this change? I suspect there
will likely be a noticeable impact some some tests.
include/linux/netdevice.h | 19 ++-----------------
net/core/dev.c | 21 +++++++++++++++++++++
2 files changed, 23 insertions(+), 17 deletions(-)
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index dcf6ec2..f650d16 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -2546,14 +2546,7 @@ static inline void netif_tx_wake_all_queues(struct net_device *dev)
}
}
-static inline void netif_tx_stop_queue(struct netdev_queue *dev_queue)
-{
- if (WARN_ON(!dev_queue)) {
- pr_info("netif_stop_queue() cannot be called before register_netdev()\n");
- return;
- }
- set_bit(__QUEUE_STATE_DRV_XOFF, &dev_queue->state);
-}
+void netif_tx_stop_queue(struct netdev_queue *dev_queue);
It looks to me like most of the overhead for this function is the
WARN_ON. Without that function would just be the "lock orb".
The question I would have is why do we need the WARN_ON? Why not let
any drivers that call netif_stop_queue before the netdev is registered
take the NULL pointer dereference? The would likely learn real quick
not to do that and a NULL pointer deference is fairly easy to debug.
You could probably even just replace the WARN_ON with a comment that if
you get a NULL pointer dereference here you probably called it before
register_netdev.
/**
* netif_stop_queue - stop transmitted packets
@@ -2567,15 +2560,7 @@ static inline void netif_stop_queue(struct net_device *dev)
netif_tx_stop_queue(netdev_get_tx_queue(dev, 0));
}
-static inline void netif_tx_stop_all_queues(struct net_device *dev)
-{
- unsigned int i;
-
- for (i = 0; i < dev->num_tx_queues; i++) {
- struct netdev_queue *txq = netdev_get_tx_queue(dev, i);
- netif_tx_stop_queue(txq);
- }
-}
+void netif_tx_stop_all_queues(struct net_device *dev);
static inline bool netif_tx_queue_stopped(const struct netdev_queue *dev_queue)
{
This is usually slow path for most device drivers so it should fine to
uninline.
diff --git a/net/core/dev.c b/net/core/dev.c
index 962ee9d..569031f 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6261,6 +6261,27 @@ static int netif_alloc_netdev_queues(struct net_device *dev)
return 0;
}
+void netif_tx_stop_queue(struct netdev_queue *dev_queue)
+{
+ if (WARN_ON(!dev_queue)) {
+ pr_info("netif_stop_queue() cannot be called before register_netdev()\n");
+ return;
+ }
+ set_bit(__QUEUE_STATE_DRV_XOFF, &dev_queue->state);
+}
+EXPORT_SYMBOL(netif_tx_stop_queue);
+
One thing I noticed on reviewing the assembly above was that you should
probably wrap the !dev_queue check in an unlikely. It would save you
some unnecessary jumps instructions.
+void netif_tx_stop_all_queues(struct net_device *dev)
+{
+ unsigned int i;
+
+ for (i = 0; i < dev->num_tx_queues; i++) {
+ struct netdev_queue *txq = netdev_get_tx_queue(dev, i);
+ netif_tx_stop_queue(txq);
+ }
+}
+EXPORT_SYMBOL(netif_tx_stop_all_queues);
+
/**
* register_netdevice - register a network device
* @dev: device to register
--
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