netdev - Re: [PATCH v2 net-next 3/3] net: Use backlog-NAPI to clean up the defer

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Date: Mon, 26 Feb 2024 12:59:22 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Jakub Kicinski <kuba@...nel.org>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Wander Lairson Costa <wander@...hat.com>,
	Yan Zhai <yan@...udflare.com>
Subject: Re: [PATCH v2 net-next 3/3] net: Use backlog-NAPI to clean up the
 defer_list.

On 2024-02-23 18:02:57 [-0800], Jakub Kicinski wrote:
> On Wed, 21 Feb 2024 18:00:13 +0100 Sebastian Andrzej Siewior wrote:
> > +	if (use_backlog_threads()) {
> > +		rps_lock_irqsave(sd, &flags);
> > +
> > +		if (!__test_and_set_bit(NAPI_STATE_SCHED, &sd->backlog.state))
> > +			napi_schedule_rps(sd);
> 
> Why are you calling napi_schedule_rps() here?
> Just do __napi_schedule_irqoff(&sd->backlog);
Looking at it, __napi_schedule_irqoff() is enough here given that we are
already in the use_backlog_threads() case.

> Then you can move the special case inside napi_schedule_rps()
> into the if (sd != mysd) block.

Okay.

> > +		rps_unlock_irq_restore(sd, &flags);
> 
> Also not sure if the lock helpers should still be called RPS since they
> also protect state on non-RPS configs now.

They protect the list in input_pkt_queue and the NAPI state. It is just
in the !RPS case it is always CPU-local and the lock is avoided (while
interrupts are still disabled/ enabled).

What about
	input_queue_lock_irq_save()
	input_queue_lock_irq_disable()
	input_queue_lock_irq_restore()
	input_queue_lock_irq_enable()
? 

Sebastian