netdev - Re: NAPI rescheduling and the delay caused by it

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Date:	Wed, 04 Dec 2013 12:41:58 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Zoltan Kiss <zoltan.kiss@...rix.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	Malcolm Crossley <malcolm.crossley@...rix.com>,
	Jonathan Davies <Jonathan.Davies@...citrix.com>,
	Paul Durrant <Paul.Durrant@...rix.com>,
	Wei Liu <wei.liu2@...rix.com>,
	Ian Campbell <Ian.Campbell@...rix.com>
Subject: Re: NAPI rescheduling and the delay caused by it

On Wed, 2013-12-04 at 18:55 +0000, Zoltan Kiss wrote:

> So, my questions are:
> - why is NAPI rescheduled on an another CPU?
> - why does it cause a 3-4 milisec delay?

NAPI can not be scheduled on another cpu.

But at the time of napi_schedule() call, napi_struct can be already be
scheduled by another cpu.

( NAPI_STATE_SCHED bit already set)

So I would say something made the 'other' cpu non responsive fast enough
to softirq events being ready for service.

(Another wakeup happened 3-4 millisec later)

Really, I suspect your usage of netif_wake_queue() is simply wrong.

Check why we have netif_rx() and netif_rx_ni() variants.

And ask yourself if xenvif_notify_tx_completion() is correct, being
called from process context.

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