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Date:   Wed, 29 Aug 2018 11:44:39 +0200
From:   Gregory CLEMENT <gregory.clement@...tlin.com>
To:     Jisheng Zhang <Jisheng.Zhang@...aptics.com>
Cc:     <thomas.petazzoni@...tlin.com>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Andrew Lunn <andrew@...n.ch>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 5/5] net: mvneta: reduce smp_processor_id() calling in mvneta_tx_done_gbe

Hi Jisheng,
 
 On mer., août 29 2018, Jisheng Zhang <Jisheng.Zhang@...aptics.com> wrote:

> In the loop of mvneta_tx_done_gbe(), we call the smp_processor_id()
> each time, move the call out of the loop to optimize the code a bit.
>
> Before the patch, the loop looks like(under arm64):
>
>         ldr     x1, [x29,#120]
>         ...
>         ldr     w24, [x1,#36]
>         ...
>         bl      0 <_raw_spin_lock>
>         str     w24, [x27,#132]
>         ...
>
> After the patch, the loop looks like(under arm64):
>
>         ...
>         bl      0 <_raw_spin_lock>
>         str     w23, [x28,#132]
>         ...
> where w23 is loaded so be ready before the loop.
>
> From another side, mvneta_tx_done_gbe() is called from mvneta_poll()
> which is in non-preemptible context, so it's safe to call the
> smp_processor_id() function once.

This improvement should go to net-next. Besides this patch looks nice:

Reviewed-by: Gregory CLEMENT <gregory.clement@...tlin.com>

Thanks,

Gregory


>
> Signed-off-by: Jisheng Zhang <Jisheng.Zhang@...aptics.com>
> ---
>  drivers/net/ethernet/marvell/mvneta.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
> index 7d98f7828a30..62e81e267e13 100644
> --- a/drivers/net/ethernet/marvell/mvneta.c
> +++ b/drivers/net/ethernet/marvell/mvneta.c
> @@ -2507,12 +2507,13 @@ static void mvneta_tx_done_gbe(struct mvneta_port *pp, u32 cause_tx_done)
>  {
>  	struct mvneta_tx_queue *txq;
>  	struct netdev_queue *nq;
> +	int cpu = smp_processor_id();
>  
>  	while (cause_tx_done) {
>  		txq = mvneta_tx_done_policy(pp, cause_tx_done);
>  
>  		nq = netdev_get_tx_queue(pp->dev, txq->id);
> -		__netif_tx_lock(nq, smp_processor_id());
> +		__netif_tx_lock(nq, cpu);
>  
>  		if (txq->count)
>  			mvneta_txq_done(pp, txq);
> -- 
> 2.18.0
>

-- 
Gregory Clement, Bootlin
Embedded Linux and Kernel engineering
http://bootlin.com

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