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Message-ID: <Y6AnEWWd7DQg0b6o@kroah.com>
Date: Mon, 19 Dec 2022 09:55:45 +0100
From: Greg KH <greg@...ah.com>
To: Leesoo Ahn <lsahn@...eel.net>
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 Mon, Dec 19, 2022 at 05:09:21PM +0900, Leesoo Ahn wrote:
>
> 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.
Ok, great! Fix up the patch based on the review comments and add this
information to the changelog as well.
thanks,
greg k-h
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