linux-kernel - Re: [net-next PATCH 2/3] net: stmmac: improve TX timer arm logic

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Message-ID: <0a62a2b6bff4fd01065e0de6a8703c96e344f1dc.camel@axis.com>
Date:   Fri, 29 Sep 2023 12:38:48 +0000
From:   Vincent Whitchurch <Vincent.Whitchurch@...s.com>
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Subject: Re: [net-next PATCH 2/3] net: stmmac: improve TX timer arm logic

On Fri, 2023-09-22 at 13:12 +0200, Christian Marangi wrote:
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> index 9201ed778ebc..14bf6fae6662 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -2994,13 +2994,25 @@ static void stmmac_tx_timer_arm(struct stmmac_priv *priv, u32 queue)
>  {
>  	struct stmmac_tx_queue *tx_q = &priv->dma_conf.tx_queue[queue];
>  	u32 tx_coal_timer = priv->tx_coal_timer[queue];
> +	struct stmmac_channel *ch;
> +	struct napi_struct *napi;
>  
> 
>  	if (!tx_coal_timer)
>  		return;
>  
> 
> -	hrtimer_start(&tx_q->txtimer,
> -		      STMMAC_COAL_TIMER(tx_coal_timer),
> -		      HRTIMER_MODE_REL);
> +	ch = &priv->channel[tx_q->queue_index];
> +	napi = tx_q->xsk_pool ? &ch->rxtx_napi : &ch->tx_napi;
> +
> +	/* Arm timer only if napi is not already scheduled.
> +	 * Try to cancel any timer if napi is scheduled, timer will be armed
> +	 * again in the next scheduled napi.
> +	 */
> +	if (unlikely(!napi_is_scheduled(napi)))
> +		hrtimer_start(&tx_q->txtimer,
> +			      STMMAC_COAL_TIMER(tx_coal_timer),
> +			      HRTIMER_MODE_REL);
> +	else
> +		hrtimer_try_to_cancel(&tx_q->txtimer);

When this function is called from within the napi poll function
(stmmac_tx_clean()), NAPI_STATE_SCHED will always be set and so after
this patch the "We still have pending packets, let's call for a new
scheduling" logic will never start the timer.  Was that really
intentional?