netdev - Re: [PATCH] softirq: let ksoftirqd do its job

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Date:   Thu, 1 Sep 2016 14:48:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        David Miller <davem@...emloft.net>,
        Rik van Riel <riel@...hat.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH] softirq: let ksoftirqd do its job

On Thu, Sep 01, 2016 at 02:38:59PM +0200, Jesper Dangaard Brouer wrote:
> On Thu, 1 Sep 2016 14:29:25 +0200
> Jesper Dangaard Brouer <brouer@...hat.com> wrote:
> 
> > On Thu, 1 Sep 2016 13:53:56 +0200
> > Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > > On Thu, Sep 01, 2016 at 01:02:31PM +0200, Jesper Dangaard Brouer wrote:  
> > > >    PID  S  %CPU     TIME+  COMMAND
> > > >      3  R  50.0  29:02.23  ksoftirqd/0
> > > >  10881  R  10.7   1:01.61  udp_sink
> > > >  10837  R  10.0   1:05.20  udp_sink
> > > >  10852  S  10.0   1:01.78  udp_sink
> > > >  10862  R  10.0   1:05.19  udp_sink
> > > >  10844  S   9.7   1:01.91  udp_sink
> > > > 
> > > > This is strange, why is ksoftirqd/0 getting 50% of the CPU time???    
> > > 
> > > Do you run your udp_sink thingy in a cpu-cgroup?  
> > 
> > That was also Paolo's feedback (IRC).  I'm not aware of it, but it
> > might be some distribution (Fedora 22) default thing.
> 
> Correction, on the server-under-test, I'm actually running RHEL7.2
> 
> 
> > How do I verify/check if I have enabled a cpu-cgroup?
> 
> Hannes says I can look in "/proc/self/cgroup"
> 
>  $ cat /proc/self/cgroup
>  7:net_cls:/
>  6:blkio:/
>  5:devices:/
>  4:perf_event:/
>  3:cpu,cpuacct:/
>  2:cpuset:/
>  1:name=systemd:/user.slice/user-1000.slice/session-c1.scope
>  
> And that "/" indicate I've not enabled cgroups, right?

Mostly so. I think RHEL/Fedora has SCHED_AUTOGROUP enabled, and you can
find that through:

cat /proc/self/autogroup

And disable with the noautogroup boot param, or:

echo 0 > /proc/sys/kernel/sched_autogroup_enabled

although this latter will leave the current state intact while avoiding
creation of any further autogroups iirc.