netdev - Re: [PATCH v3] usbnet: optimize usbnet

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Date:   Thu, 22 Dec 2022 12:13:44 +0100
From:   Paolo Abeni <pabeni@...hat.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>, Greg KH <greg@...ah.com>,
        netdev@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] usbnet: optimize usbnet_bh() to reduce CPU load

On Wed, 2022-12-21 at 16:59 +0900, Leesoo Ahn wrote:
> The current source pushes skb into dev->done queue by calling
> skb_queue_tail() and then pop it by calling skb_dequeue() to branch to
> rx_cleanup state for freeing urb/skb in usbnet_bh(). It takes extra CPU
> load, 2.21% (skb_queue_tail) as follows.
> 
> -   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
> 
> To reduce the extra CPU load use return values jumping to rx_cleanup
> state directly to free them instead of calling skb_queue_tail() and
> skb_dequeue() for push/pop respectively.
> 
> -    7.87%     0.25%  swapper          [k] usbnet_bh
>    - 7.62% usbnet_bh
>       - 4.81% skb_dequeue
>            4.74% _raw_spin_unlock_irqrestore
>       - 1.75% consume_skb
>          - 0.98% kfree_skbmem
>               0.78% kmem_cache_free
>            0.58% skb_release_data
>         0.53% smsc95xx_rx_fixup
> 
> Signed-off-by: Leesoo Ahn <lsahn@...eel.net>
> ---
> v3:
>   - Replace return values with proper -ERR values in rx_process()
> 
> v2:
>   - Replace goto label with return statement to reduce goto entropy
>   - Add CPU load information by perf in commit message
> 
> v1 at:
>   https://patchwork.kernel.org/project/netdevbpf/patch/20221217161851.829497-1-lsahn@ooseel.net/

This looks like net-next material.

We have already submitted the networking pull request to Linus
for v6.2 and therefore net-next is closed for new drivers, features,
code refactoring and optimizations. We are currently accepting
bug fixes only.

Please repost when net-next reopens after Jan 2nd, including the
expected 'net-next' tag into the subject line.

RFC patches sent for review only are obviously welcome at any time.

[...]

> @@ -1528,13 +1526,14 @@ static void usbnet_bh (struct timer_list *t)
>  		entry = (struct skb_data *) skb->cb;
>  		switch (entry->state) {
>  		case rx_done:
> -			entry->state = rx_cleanup;
> -			rx_process (dev, skb);
> +			if (rx_process(dev, skb))
> +				goto cleanup;

You can avoid this additional label (which is a little confusing inside
a switch) factoring out a usb_free_skb(skb) helper and calling it here
and under the rx_cleanup case.

Cheers,

Paolo