netdev - Re: [PATCH] usbnet: jump to rx_cleanup case instead of calling skb_queue

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Date:   Mon, 19 Dec 2022 16:41:16 +0900
From:   Leesoo Ahn <lsahn@...eel.net>
To:     Greg KH <greg@...ah.com>
Cc:     Oliver Neukum <oneukum@...e.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usbnet: jump to rx_cleanup case instead of calling
 skb_queue_tail


On 22. 12. 18. 17:55, Greg KH wrote:
> On Sun, Dec 18, 2022 at 01:18:51AM +0900, Leesoo Ahn wrote:
>> The current source pushes skb into dev->done queue by calling
>> skb_queue_tail() and then, call skb_dequeue() to pop for rx_cleanup state
>> to free urb and skb next in usbnet_bh().
>> It wastes CPU resource with extra instructions. Instead, use return values
>> jumping to rx_cleanup case directly to free them. Therefore calling
>> skb_queue_tail() and skb_dequeue() is not necessary.
>>
>> The follows are just showing difference between calling skb_queue_tail()
>> and using return values jumping to rx_cleanup state directly in usbnet_bh()
>> in Arm64 instructions with perf tool.
>>
>> ----------- calling skb_queue_tail() -----------
>>         │     if (!(dev->driver_info->flags & FLAG_RX_ASSEMBLE))
>>    7.58 │248:   ldr     x0, [x20, #16]
>>    2.46 │24c:   ldr     w0, [x0, #8]
>>    1.64 │250: ↑ tbnz    w0, #14, 16c
>>         │     dev->net->stats.rx_errors++;
>>    0.57 │254:   ldr     x1, [x20, #184]
>>    1.64 │258:   ldr     x0, [x1, #336]
>>    2.65 │25c:   add     x0, x0, #0x1
>>         │260:   str     x0, [x1, #336]
>>         │     skb_queue_tail(&dev->done, skb);
>>    0.38 │264:   mov     x1, x19
>>         │268:   mov     x0, x21
>>    2.27 │26c: → bl      skb_queue_tail
>>    0.57 │270: ↑ b       44    // branch to call skb_dequeue()
>>
>> ----------- jumping to rx_cleanup state -----------
>>         │     if (!(dev->driver_info->flags & FLAG_RX_ASSEMBLE))
>>    1.69 │25c:   ldr     x0, [x21, #16]
>>    4.78 │260:   ldr     w0, [x0, #8]
>>    3.28 │264: ↑ tbnz    w0, #14, e4    // jump to 'rx_cleanup' state
>>         │     dev->net->stats.rx_errors++;
>>    0.09 │268:   ldr     x1, [x21, #184]
>>    2.72 │26c:   ldr     x0, [x1, #336]
>>    3.37 │270:   add     x0, x0, #0x1
>>    0.09 │274:   str     x0, [x1, #336]
>>    0.66 │278: ↑ b       e4    // branch to 'rx_cleanup' state
> Interesting, but does this even really matter given the slow speed of
> the USB hardware?

It doesn't if USB hardware has slow speed but in software view, it's 
still worth avoiding calling skb_queue_tail() and skb_dequeue() which 
work with spinlock, if possible.


>> Signed-off-by: Leesoo Ahn <lsahn@...eel.net>
>> ---
>>   drivers/net/usb/usbnet.c | 11 ++++++-----
>>   1 file changed, 6 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
>> index 64a9a80b2309..924392a37297 100644
>> --- a/drivers/net/usb/usbnet.c
>> +++ b/drivers/net/usb/usbnet.c
>> @@ -555,7 +555,7 @@ static int rx_submit (struct usbnet *dev, struct urb *urb, gfp_t flags)
>>   
>>   /*-------------------------------------------------------------------------*/
>>   
>> -static inline void rx_process (struct usbnet *dev, struct sk_buff *skb)
>> +static inline int rx_process(struct usbnet *dev, struct sk_buff *skb)
>>   {
>>   	if (dev->driver_info->rx_fixup &&
>>   	    !dev->driver_info->rx_fixup (dev, skb)) {
>> @@ -576,11 +576,11 @@ static inline void rx_process (struct usbnet *dev, struct sk_buff *skb)
>>   		netif_dbg(dev, rx_err, dev->net, "rx length %d\n", skb->len);
>>   	} else {
>>   		usbnet_skb_return(dev, skb);
>> -		return;
>> +		return 0;
>>   	}
>>   
>>   done:
>> -	skb_queue_tail(&dev->done, skb);
>> +	return -1;
> Don't make up error numbers, this makes it look like this failed, not
> succeeded.  And if this failed, give it a real error value.
>
> thanks,
>
> greg k-h

Best regards,
Leesoo