linux-kernel - Re: [RFC PATCH 2/3] chelsio: cxgb: Move slow interrupt handling to threaded irqs

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Date:   Mon, 11 Jan 2021 11:38:47 +0100
From:   "Sebastian A. Siewior" <bigeasy@...utronix.de>
To:     "Ahmed S. Darwish" <a.darwish@...utronix.de>
Cc:     Rahul Lakkireddy <rahul.lakkireddy@...lsio.com>,
        Rohit Maheshwari <rohitm@...lsio.com>,
        Vinay Kumar Yadav <vinay.yadav@...lsio.com>,
        Vishal Kulkarni <vishal@...lsio.com>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 2/3] chelsio: cxgb: Move slow interrupt handling to
 threaded irqs

On 2020-12-24 14:11:47 [+0100], Ahmed S. Darwish wrote:
> --- a/drivers/net/ethernet/chelsio/cxgb/cxgb2.c
> +++ b/drivers/net/ethernet/chelsio/cxgb/cxgb2.c
> @@ -211,9 +211,9 @@ static int cxgb_up(struct adapter *adapter)
>  	t1_interrupts_clear(adapter);
>  
>  	adapter->params.has_msi = !disable_msi && !pci_enable_msi(adapter->pdev);
> -	err = request_irq(adapter->pdev->irq, t1_interrupt,
> -			  adapter->params.has_msi ? 0 : IRQF_SHARED,
> -			  adapter->name, adapter);
> +	err = request_threaded_irq(adapter->pdev->irq, t1_irq, t1_irq_thread,
> +				   adapter->params.has_msi ? 0 : IRQF_SHARED,
> +				   adapter->name, adapter);
>  	if (err) {
>  		if (adapter->params.has_msi)
>  			pci_disable_msi(adapter->pdev);
> diff --git a/drivers/net/ethernet/chelsio/cxgb/sge.c b/drivers/net/ethernet/chelsio/cxgb/sge.c
> index d6df1a87db0b..f1c402f6b889 100644
> --- a/drivers/net/ethernet/chelsio/cxgb/sge.c
> +++ b/drivers/net/ethernet/chelsio/cxgb/sge.c
> @@ -1626,11 +1626,10 @@ int t1_poll(struct napi_struct *napi, int budget)
>  	return work_done;
>  }
>  
> -irqreturn_t t1_interrupt(int irq, void *data)
> +irqreturn_t t1_irq(int irq, void *data)
>  {
>  	struct adapter *adapter = data;
>  	struct sge *sge = adapter->sge;
> -	int handled;
>  
>  	if (likely(responses_pending(adapter))) {
>  		writel(F_PL_INTR_SGE_DATA, adapter->regs + A_PL_CAUSE);
> @@ -1645,9 +1644,19 @@ irqreturn_t t1_interrupt(int irq, void *data)
>  				napi_enable(&adapter->napi);
>  			}
>  		}
> +
>  		return IRQ_HANDLED;
>  	}
>  
> +	return IRQ_WAKE_THREAD;
> +}
> +
> +irqreturn_t t1_irq_thread(int irq, void *data)
> +{
> +	struct adapter *adapter = data;
> +	struct sge *sge = adapter->sge;
> +	int handled;
> +
>  	spin_lock(&adapter->async_lock);
>  	handled = t1_slow_intr_handler(adapter);
>  	spin_unlock(&adapter->async_lock);

This does not work in general, it might work in the MSI case but does
not work for the LEVEL interrupt case: The interrupt remains active
because it has not been ACKed. Chances are that the threaded handler
never gets scheduled because interrupt is still pending and t1_irq()
gets invoked right away.
For that reason, the primary must either mask the interrupt source or
use IRQF_ONESHOT to mask the interrupt line until the threaded handler
is done.

If you look at t1_elmer0_ext_intr() it disables F_PL_INTR_EXT before the
worker scheduled so the interrupt does not trigger again.
The worker then does what ever is needed (t1_elmer0_ext_intr_handler)
and then ACKs F_PL_INTR_EXT and enables F_PL_INTR_EXT again so it may
trigger an interrupt again.

Sebastian