netdev - Re: [PATCH net-next 1/2] net: tap: replay while() loop with % operator in tap_get

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Message-Id: <20171211.120246.1487812563373661375.davem@davemloft.net>
Date:   Mon, 11 Dec 2017 12:02:46 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     stephen@...workplumber.org
Cc:     cugyly@....com, netdev@...r.kernel.org,
        Linyu.Yuan@...atel-sbell.com.cn
Subject: Re: [PATCH net-next 1/2] net: tap: replay while() loop with %
 operator in tap_get_queue()

From: Stephen Hemminger <stephen@...workplumber.org>
Date: Mon, 11 Dec 2017 08:58:11 -0800

> On Mon, 11 Dec 2017 21:26:41 +0800
> yuan linyu <cugyly@....com> wrote:
> 
>> From: yuan linyu <Linyu.Yuan@...atel-sbell.com.cn>
>> 
>> Signed-off-by: yuan linyu <Linyu.Yuan@...atel-sbell.com.cn>
>> ---
>>  drivers/net/tap.c | 6 +-----
>>  1 file changed, 1 insertion(+), 5 deletions(-)
>> 
>> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
>> index 0a886fda..78900a0 100644
>> --- a/drivers/net/tap.c
>> +++ b/drivers/net/tap.c
>> @@ -275,11 +275,7 @@ static struct tap_queue *tap_get_queue(struct tap_dev *tap,
>>  
>>  	if (likely(skb_rx_queue_recorded(skb))) {
>>  		rxq = skb_get_rx_queue(skb);
>> -
>> -		while (unlikely(rxq >= numvtaps))
>> -			rxq -= numvtaps;
>> -
>> -		queue = rcu_dereference(tap->taps[rxq]);
>> +		queue = rcu_dereference(tap->taps[rxq % numvtaps]);
>>  		goto out;
>>  	}
>>  
> 
> Modulus is slower than the loop.

Agreed, the loop is there intentionally.  We do this in other areas of
the kernel as well.