netdev - Re: [PATCH] atl1c: optimize rx loop

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Date:   Fri, 19 Mar 2021 05:15:35 +0100
From:   Willy Tarreau <w@....eu>
To:     Sieng Piaw Liew <liew.s.piaw@...il.com>
Cc:     chris.snook@...il.com, davem@...emloft.net, kuba@...nel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] atl1c: optimize rx loop

On Fri, Mar 19, 2021 at 12:04:47PM +0800, Sieng Piaw Liew wrote:
> Remove this trivial bit of inefficiency from the rx receive loop,
> results in increase of a few Mbps in iperf3. Tested on Intel Core2
> platform.
> 
> Signed-off-by: Sieng Piaw Liew <liew.s.piaw@...il.com>
> ---
>  drivers/net/ethernet/atheros/atl1c/atl1c_main.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/drivers/net/ethernet/atheros/atl1c/atl1c_main.c b/drivers/net/ethernet/atheros/atl1c/atl1c_main.c
> index 3f65f2b370c5..b995f9a0479c 100644
> --- a/drivers/net/ethernet/atheros/atl1c/atl1c_main.c
> +++ b/drivers/net/ethernet/atheros/atl1c/atl1c_main.c
> @@ -1796,9 +1796,7 @@ static void atl1c_clean_rx_irq(struct atl1c_adapter *adapter,
>  	struct atl1c_recv_ret_status *rrs;
>  	struct atl1c_buffer *buffer_info;
>  
> -	while (1) {
> -		if (*work_done >= work_to_do)
> -			break;
> +	while (*work_done < work_to_do) {

It should not change anything, or only based on the compiler's optimization
and should not result in a measurable difference because what it does is
exactly the same. Have you really compared the compiled output code to
explain the difference ? I strongly suspect you'll find no difference at
all.

Thus for me it's certainly not an optimization, it could be qualified as
a cleanup to improve code readability however.

Willy