linux-kernel - Re: [PATCH AUTOSEL 5.10 016/137] net: Treat __napi_schedule_irqoff() as __napi_schedule() on PREEMPT

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Date:   Mon, 12 Jul 2021 23:52:59 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Juri Lelli <juri.lelli@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 5.10 016/137] net: Treat __napi_schedule_irqoff()
 as __napi_schedule() on PREEMPT_RT

Hi!

> __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.

I see this is queued to kernels as old as 4.4... Is it good idea?
PREEMPT_RT is not usable there without extra patches, so it does not
really fix anything user visible....

Best regards,
								Pavel
								
> index 0c9ce36afc8c..2fdf30eefc59 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -6433,11 +6433,18 @@ 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);
>  

-- 
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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