netdev - Re: [RFC PATCH 0/2] net: threadable napi poll loop

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Date:	Tue, 10 May 2016 17:01:21 -0400
From:	Rik van Riel <riel@...hat.com>
To:	David Miller <davem@...emloft.net>
Cc:	pabeni@...hat.com, eric.dumazet@...il.com, netdev@...r.kernel.org,
	edumazet@...gle.com, jiri@...lanox.com, daniel@...earbox.net,
	ast@...mgrid.com, aduyck@...antis.com, tom@...bertland.com,
	peterz@...radead.org, mingo@...nel.org, hannes@...essinduktion.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/2] net: threadable napi poll loop

On Tue, 2016-05-10 at 16:52 -0400, David Miller wrote:
> From: Rik van Riel <riel@...hat.com>
> Date: Tue, 10 May 2016 16:50:56 -0400
> 
> > On Tue, 2016-05-10 at 16:45 -0400, David Miller wrote:
> >> From: Paolo Abeni <pabeni@...hat.com>
> >> Date: Tue, 10 May 2016 22:22:50 +0200
> >> 
> >> > On Tue, 2016-05-10 at 09:08 -0700, Eric Dumazet wrote:
> >> >> On Tue, 2016-05-10 at 18:03 +0200, Paolo Abeni wrote:
> >> >> 
> >> >> > If a single core host is under network flood, i.e. ksoftirqd
> is
> >> >> > scheduled and it eventually (after processing ~640 packets)
> will
> >> let the
> >> >> > user space process run. The latter will execute a syscall to
> >> receive a
> >> >> > packet, which will have to disable/enable bh at least once
> and
> >> that will
> >> >> > cause the processing of another ~640 packets. To receive a
> >> single packet
> >> >> > in user space, the kernel has to process more than one
> thousand
> >> packets.
> >> >> 
> >> >> Looks you found the bug then. Have you tried to fix it ?
> >>  ...
> >> > The ksoftirq and the local_bh_enable() design are the root of
> the
> >> > problem, they need to be touched/affected to solve it.
> >> 
> >> That's not what I read from your description, processing 640
> packets
> >> before going to ksoftirqd seems to the be the absolute root
> problem.
> > 
> > What would a fix for that look like?
> > 
> > Keep track of the number of processed incoming packets,
> > and the number of packets handed off, and defer to
> > ksoftirqd earlier if the statistics suggest packets are
> > getting dropped on the floor?
> 
> Not by packet count but by something more easily to measure and
> scalable to fairness like processing time.

I need to get back to fixing irq & softirq time accounting,
which does not currently work correctly in all time keeping
modes...

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