linux-kernel - Re: net: macb: can macb use __napi_schedule_irqoff() instead of __napi

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Date:   Mon, 11 Jan 2021 17:09:24 -0500
From:   Paul Thomas <pthomas8589@...il.com>
To:     Heiner Kallweit <hkallweit1@...il.com>
Cc:     Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Claudiu Beznea <claudiu.beznea@...rochip.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        netdev@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-riscv@...ts.infradead.org
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 <hkallweit1@...il.com> 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
> >
>