netdev - Re: [PATCH net-next] net: Treat __napi_schedule_irqoff() as __napi_schedule() on PREEMPT

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Date:   Thu, 13 May 2021 07:12:16 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     netdev@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>, sassmann@...hat.com,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH net-next] net: Treat __napi_schedule_irqoff() as
 __napi_schedule() on PREEMPT_RT

Hi,

On 12/05/21 23:43, Sebastian Andrzej Siewior wrote:
> __napi_schedule_irqoff() is an optimized version of __napi_schedule()
> which can be used where it is known that interrupts are disabled,
> e.g. in interrupt-handlers, spin_lock_irq() sections or hrtimer
> callbacks.
> 
> On PREEMPT_RT enabled kernels this assumptions is not true. Force-
> threaded interrupt handlers and spinlocks are not disabling interrupts
> and the NAPI hrtimer callback is forced into softirq context which runs
> with interrupts enabled as well.
> 
> Chasing all usage sites of __napi_schedule_irqoff() is a whack-a-mole
> game so make __napi_schedule_irqoff() invoke __napi_schedule() for
> PREEMPT_RT kernels.
> 
> The callers of ____napi_schedule() in the networking core have been
> audited and are correct on PREEMPT_RT kernels as well.
> 
> Reported-by: Juri Lelli <juri.lelli@...hat.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
> Alternatively __napi_schedule_irqoff() could be #ifdef'ed out on RT and
> an inline provided which invokes __napi_schedule().
> 
> This was not chosen as it creates #ifdeffery all over the place and with
> the proposed solution the code reflects the documentation consistently
> and in one obvious place.
> 
>  net/core/dev.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 222b1d322c969..febb23708184e 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -6501,11 +6501,18 @@ EXPORT_SYMBOL(napi_schedule_prep);
>   * __napi_schedule_irqoff - schedule for receive
>   * @n: entry to schedule
>   *
> - * Variant of __napi_schedule() assuming hard irqs are masked
> + * Variant of __napi_schedule() assuming hard irqs are masked.
> + *
> + * On PREEMPT_RT enabled kernels this maps to __napi_schedule()
> + * because the interrupt disabled assumption might not be true
> + * due to force-threaded interrupts and spinlock substitution.
>   */
>  void __napi_schedule_irqoff(struct napi_struct *n)
>  {
> -	____napi_schedule(this_cpu_ptr(&softnet_data), n);
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> +		____napi_schedule(this_cpu_ptr(&softnet_data), n);
> +	else
> +		__napi_schedule(n);
>  }
>  EXPORT_SYMBOL(__napi_schedule_irqoff);

Thanks for the patch!

Reviewed-by: Juri Lelli <juri.lelli@...hat.com>

Best,
Juri