netdev - Re: [PATCH net-next 0/5] implement kthread based napi poll

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Date:   Fri, 2 Oct 2020 15:49:26 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Wei Wang <weiwan@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Paolo Abeni <pabeni@...hat.com>, Felix Fietkau <nbd@....name>
Subject: Re: [PATCH net-next 0/5] implement kthread based napi poll

On Fri, 2 Oct 2020 09:56:31 +0200 Eric Dumazet wrote:
> On Thu, Oct 1, 2020 at 10:26 PM Jakub Kicinski <kuba@...nel.org> wrote:
> >
> > On Thu, 1 Oct 2020 09:52:45 +0200 Eric Dumazet wrote:  
> 
> > > The unique work queue is a problem on server class platforms, with
> > > NUMA placement.
> > > We now have servers with NIC on different NUMA nodes.  
> >
> > Are you saying that the wq code is less NUMA friendly than unpinned
> > threads?  
> 
> Yes this is what I am saying.
> 
> Using a single and shared wq wont allow you to make sure :
> - work for NIC0 attached on NUMA node#0 will be using CPUS belonging to node#0
> - work for NIC1 attached on NUMA node#1 will be using CPUS belonging to node#1
> 
> 
> The only way you can tune things with a single wq is tweaking a single cpumask,
> that we can change with /sys/devices/virtual/workqueue/{wqname}/cpumask
> The same for the nice value with  /sys/devices/virtual/workqueue/{wqname}/nice.
> 
> In contrast, having kthreads let you tune things independently, if needed.
> 
> Even with a single NIC, you can still need isolation between queues.
> We have queues dedicated to a certain kind of traffic/application.
> 
> The work queue approach would need to be able to create/delete
> independent workqueues.
> But we tested the workqueue with a single NIC and our results gave to
> kthreads a win over the work queue.

Not according to the results Wei posted last night..

> Really, wq concept might be a nice abstraction when each work can be
> running for arbitrary durations,
> and arbitrary numbers of cpus, but with the NAPI model of up to 64
> packets at a time, and a fixed number of queues,

In my experiments the worker threads get stalled sooner or later. 
And unless there is some work stealing going on latency spikes follow.

I would also not discount the variability in processing time. For a
budget of 64 the processing can take 0-500us per round, not counting
outliers.

> we should not add the work queue overhead.

Does this mean you're going to be against the (more fleshed out)
work queue implementation?