netdev - Re: [PATCH net-next 4/4] net: dev: Make rps

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Date:   Wed, 2 Feb 2022 08:47:35 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH net-next 4/4] net: dev: Make rps_lock() disable
 interrupts.

On Wed,  2 Feb 2022 13:28:48 +0100 Sebastian Andrzej Siewior wrote:
>  		/* Schedule NAPI for backlog device
>  		 * We can use non atomic operation since we own the queue lock
> +		 * PREEMPT_RT needs to disable interrupts here for
> +		 * synchronisation needed in napi_schedule.
>  		 */
> +		if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +			local_irq_disable();
> +
>  		if (!__test_and_set_bit(NAPI_STATE_SCHED, &sd->backlog.state)) {
>  			if (!rps_ipi_queued(sd))
>  				____napi_schedule(sd, &sd->backlog);
>  		}
> +		if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +			local_irq_enable();
>  		goto enqueue;

I think you can re-jig this a little more - rps_ipi_queued() only return
0 if sd is "local" so maybe we can call __napi_schedule_irqoff()
instead which already has the if () for PREEMPT_RT?

Maybe moving the ____napi_schedule() into rps_ipi_queued() and
renaming it to napi_schedule_backlog() or napi_schedule_rps() 
would make the code easier to follow in that case?