2021-01-11 19:50:49

by Paul Thomas

Subject: net: macb: can macb use __napi_schedule_irqoff() instead of __napi_schedule()

Hello, recently I was doing a lot of tracing/profiling to understand
an issue we were having. Anyway, during this I ran across
__napi_schedule_irqoff() where the comment in dev.c says "Variant of
__napi_schedule() assuming hard irqs are masked".

It looks like the queue_writel(queue, IDR, bp->rx_intr_mask); call
just before the __napi_schedule() call in macb_main.c is doing this
hard irq masking? So could it change to be like this?

--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c
@@ -1616,7 +1623,7 @@ static irqreturn_t macb_interrupt(int irq, void *dev_id)

if (napi_schedule_prep(&queue->napi)) {
netdev_vdbg(bp->dev, "scheduling RX softirq\n");
- __napi_schedule(&queue->napi);
+ __napi_schedule_irqoff(&queue->napi);
}
}

-Paul

2021-01-11 21:39:03

by Heiner Kallweit

Subject: Re: net: macb: can macb use __napi_schedule_irqoff() instead of __napi_schedule()

On 11.01.2021 20:45, Paul Thomas wrote:
> Hello, recently I was doing a lot of tracing/profiling to understand
> an issue we were having. Anyway, during this I ran across
> __napi_schedule_irqoff() where the comment in dev.c says "Variant of
> __napi_schedule() assuming hard irqs are masked".
>
> It looks like the queue_writel(queue, IDR, bp->rx_intr_mask); call
> just before the __napi_schedule() call in macb_main.c is doing this
> hard irq masking? So could it change to be like this?
>
It's unsafe under forced irq threading. There has been a number of
discussions about this topic in the past.

> --- a/drivers/net/ethernet/cadence/macb_main.c
> +++ b/drivers/net/ethernet/cadence/macb_main.c
> @@ -1616,7 +1623,7 @@ static irqreturn_t macb_interrupt(int irq, void *dev_id)
>
> if (napi_schedule_prep(&queue->napi)) {
> netdev_vdbg(bp->dev, "scheduling RX softirq\n");
> - __napi_schedule(&queue->napi);
> + __napi_schedule_irqoff(&queue->napi);
> }
> }
>
> -Paul
>

2021-01-11 22:12:55

by Paul Thomas

Subject: Re: net: macb: can macb use __napi_schedule_irqoff() instead of __napi_schedule()

On Mon, Jan 11, 2021 at 4:35 PM Heiner Kallweit <[email protected]> wrote:
>
> On 11.01.2021 20:45, Paul Thomas wrote:
> > Hello, recently I was doing a lot of tracing/profiling to understand
> > an issue we were having. Anyway, during this I ran across
> > __napi_schedule_irqoff() where the comment in dev.c says "Variant of
> > __napi_schedule() assuming hard irqs are masked".
> >
> > It looks like the queue_writel(queue, IDR, bp->rx_intr_mask); call
> > just before the __napi_schedule() call in macb_main.c is doing this
> > hard irq masking? So could it change to be like this?
> >
> It's unsafe under forced irq threading. There has been a number of
> discussions about this topic in the past.
OK thanks, and our use case is forced irq threading under PREEMPT_RT

-Paul

> > --- a/drivers/net/ethernet/cadence/macb_main.c
> > +++ b/drivers/net/ethernet/cadence/macb_main.c
> > @@ -1616,7 +1623,7 @@ static irqreturn_t macb_interrupt(int irq, void *dev_id)
> >
> > if (napi_schedule_prep(&queue->napi)) {
> > netdev_vdbg(bp->dev, "scheduling RX softirq\n");
> > - __napi_schedule(&queue->napi);
> > + __napi_schedule_irqoff(&queue->napi);
> > }
> > }
> >
> > -Paul
> >
>