On Sun, Dec 18, 2022 at 09:55:44AM +0100, 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?
On the other side, it is pretty nice optimization and a proof someone
read the code really carefully.
> > 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.
Note that jumps to 'done' label can be avoided now, so eventual v2 version
of that patch doesn't increase total goto entropy.
l.
> thanks,
>
> greg k-h
