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

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Date:	Wed, 11 May 2016 00:32:48 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Eric Dumazet <edumazet@...gle.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Paolo Abeni <pabeni@...hat.com>,
	netdev <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Jiri Pirko <jiri@...lanox.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Alexander Duyck <aduyck@...antis.com>,
	Tom Herbert <tom@...bertland.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/2] net: threadable napi poll loop

On 10.05.2016 23:09, Eric Dumazet wrote:
> On Tue, May 10, 2016 at 1:46 PM, Hannes Frederic Sowa
> <hannes@...essinduktion.org> wrote:
> 
>> I agree here, but I don't think this patch particularly is a lot of
>> bloat and something very interesting people can play with and extend upon.
>>
> 
> Sure, very rarely patch authors think their stuff is bloat.
> 
> I prefer to fix kernel softirq.c, or at least show me that you tried
> hard enough.
> 
> I am pretty sure that the following would work :
> 
> When ksoftirqd is scheduled, remember this in a per cpu variable
> (ksoftiqd_scheduled)
> 
> When enabling BH , do not call do_softirq() if this variable is set.
> 
> ksoftirqd would clear the variable at the right place (probably in
> run_ksoftirqd())
> 
> Sure, this might add a lot of latency regressions, but lets fix them.

Probably, yes.

We had a version which limited the number of restarts if softirqs were
invoked from local_bh_enable (so that at least timers etc. would run)
and would defer all other work to ksoftirqd. That also solved the
initial live lock problem. I do have concerns about the fairness of this
approach, but we now have to investigate this. ;)

Not only did we want to present this solely as a bugfix but also as as
performance enhancements in case of virtio (as you can see in the cover
letter). Given that a long time ago there was a tendency to remove
softirqs completely, we thought it might be very interesting, that a
threaded napi in general seems to be absolutely viable nowadays and
might offer new features.

Bye,
Hannes