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Date: Thu, 07 May 2015 10:14:42 -0700
From: Alexander Duyck <alexander.duyck@...il.com>
To: Denys Vlasenko <dvlasenk@...hat.com>,
"David S. Miller" <davem@...emloft.net>
CC: Jiri Pirko <jpirko@...hat.com>, linux-kernel@...r.kernel.org,
netdev@...r.kernel.org, netfilter-devel@...r.kernel.org
Subject: Re: [PATCH] net: deinline netif_tx_stop_queue() and netif_tx_stop_all_queues()
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@...hat.com>
> CC: David S. Miller <davem@...emloft.net>
> CC: Jiri Pirko <jpirko@...hat.com>
> CC: linux-kernel@...r.kernel.org
> CC: netdev@...r.kernel.org
> CC: netfilter-devel@...r.kernel.org
> ---
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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