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

From: Paolo Abeni <[email-masked]>
Date: Wed, 11 May 2016 11:48:24 +0200

Hi Eric,
On Tue, 2016-05-10 at 15:51 -0700, Eric Dumazet wrote:
> On Wed, 2016-05-11 at 00:32 +0200, Hannes Frederic Sowa wrote:
> 
> > 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.
> 
> Well, you did not fix the bug, you worked around by adding yet another
> layer, with another sysctl that admins or programs have to manage.
> 
> If you have a special need for virtio, do not hide it behind a 'bug fix'
> but add it as a features request.
> 
> This ksoftirqd issue is real and a fix looks very reasonable.
> 
> Please try this patch, as I had very good success with it.

Thank you for your time and your effort.

I tested your patch on the bare metal "single core" scenario, disabling
the unneeded cores with:
CPUS=`nproc`
for I in `seq 1 $CPUS`; do echo 0  >  /sys/devices/system/node/node0/cpu$I/online; done

And I got a:

[   86.925249] Broke affinity for irq <num>

for each irq number generated by a network device.

In this scenario, your patch solves the ksoftirqd issue, performing
comparable to the napi threaded patches (with a negative delta in the
noise range) and introducing a minor regression with a single flow, in
the noise range (3%).

As said in a previous mail, we actually experimented something similar,
but it felt quite hackish.

AFAICS this patch adds three more tests in the fast path and affect all
other softirq use case. I'm not sure how to check for regression there.

The napi thread patches are actually a new feature, that also fixes the
ksoftirqd issue: hunting the ksoftirqd issue has been the initial
trigger for this work. I'm sorry for not being clear enough in the cover
letter.

The napi thread patches offer additional benefits, i.e. an additional
relevant gain in the described test scenario, and do not impact on other
subsystems/kernel entities. 

I still think they are worthy, and I bet you would disagree, but could
you please articulate more which parts concern you most and/or are more
bloated ?

Thank you,

Paolo