lkml.org 
[lkml]   [2022]   [Dec]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH] usbnet: jump to rx_cleanup case instead of calling skb_queue_tail
From

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



\
 
 \ /
  Last update: 2022-12-19 09:09    [W:1.081 / U:0.272 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site