Re: [PATCH] usbnet: jump to rx_cleanup case instead of calling skb_queue_tail

[Leesoo Ahn]

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