netdev - Re: [PATCH] net: use unlikely for dql

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Date:   Wed, 25 Sep 2019 14:25:01 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     xiaolinkui <xiaolinkui@...inos.cn>
Cc:     davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH] net: use unlikely for dql_avail case

On Wed, Sep 25, 2019 at 10:40:43AM +0800, xiaolinkui wrote:
> This is an unlikely case, use unlikely() on it seems logical.
> 
> Signed-off-by: xiaolinkui <xiaolinkui@...inos.cn>

It's already here [0], but should probably rather get reverted instead
due to lack of a more elaborate reasoning on why it needs to be done
this way instead of letting compiler do it's job in this case. "Seems
logical" is never a good technical explanation. Do you have any better
analysis you performed prior to submitting the patch (twice by now)?

Thanks,
Daniel

  [0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f3acd33d840d3ea3e1233d234605c85cbbf26054

> ---
>  include/linux/netdevice.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 88292953aa6f..005f3da1b13d 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -3270,7 +3270,7 @@ static inline void netdev_tx_completed_queue(struct netdev_queue *dev_queue,
>  	 */
>  	smp_mb();
>  
> -	if (dql_avail(&dev_queue->dql) < 0)
> +	if (unlikely(dql_avail(&dev_queue->dql) < 0))
>  		return;
>  
>  	if (test_and_clear_bit(__QUEUE_STATE_STACK_XOFF, &dev_queue->state))
> -- 
> 2.17.1
> 
> 
>