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

[Leesoo Ahn]

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@xxxxxxxxxx>
> > ---
> >   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