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

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Message-ID: <d5225e3a-717e-c836-f94c-6df4f29e43ca@gmail.com>
Date:   Thu, 26 Sep 2019 15:52:29 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        xiaolinkui <xiaolinkui@...inos.cn>
Cc:     davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH] net: use unlikely for dql_avail case



On 9/25/19 5:25 AM, Daniel Borkmann wrote:
> 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)?
>

Yes, we need more details here.

We could probably save more cpu cycles checking if we can move
the smb_mb() after the dql_avail() check :)


 
> 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
>>
>>
>>