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

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Date:   Mon, 19 Dec 2022 17:09:21 +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. 19. 16:50, Greg KH wrote:
> On Mon, Dec 19, 2022 at 04:41:16PM +0900, Leesoo Ahn wrote:
>> 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.
> But can you actually measure that in either CPU load or in increased
> transfer speeds?
>
> thanks,
>
> greg k-h

I think the follows are maybe what you would be interested in. I have 
tested both case with perf on the same machine and environments, also 
modified driver code a bit to go to rx_cleanup case, not to net stack in 
a specific packet.

----- calling skb_queue_tail() -----
-   11.58%     0.26%  swapper          [k] usbnet_bh
    - 11.32% usbnet_bh
       - 6.43% skb_dequeue
            6.34% _raw_spin_unlock_irqrestore
       - 2.21% skb_queue_tail
            2.19% _raw_spin_unlock_irqrestore
       - 1.68% consume_skb
          - 0.97% kfree_skbmem
               0.80% kmem_cache_free
            0.53% skb_release_data

----- jump to rx_cleanup directly -----
-    7.62%     0.18%  swapper          [k] usbnet_bh
    - 7.44% usbnet_bh
       - 4.63% skb_dequeue
            4.57% _raw_spin_unlock_irqrestore
       - 1.76% consume_skb
          - 1.03% kfree_skbmem
               0.86% kmem_cache_free
            0.56% skb_release_data
         0.54% smsc95xx_rx_fixup

The first case takes CPU resource a bit much by the result.

Thank you for reviewing, by the way.

Best regards,
Leesoo