netdev - Re: [RFC PATCH 0/2] net:setup XPS mapping for each online CPU

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Date:   Thu, 15 Mar 2018 16:51:22 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        intel-wired-lan@...ts.osuosl.org,
        Alexander Duyck <alexander.h.duyck@...el.com>
Subject: Re: [RFC PATCH 0/2] net:setup XPS mapping for each online CPU

Hi,

On Thu, 2018-03-15 at 15:31 +0000, Eric Dumazet wrote:
> On Thu, Mar 15, 2018 at 8:08 AM Paolo Abeni <pabeni@...hat.com> wrote:
> 
> > Currently, most MQ netdevice setup the default XPS configuration mapping
> 
> 1-1
> > the first real_num_tx_queues queues and CPUs and no mapping is created for
> > the CPUs with id greater then real_num_tx_queues, if any.
> > As a result, the xmit path for unconnected sockets on such cores
> 
> experiences a
> > relevant overhead in netdev_pick_tx(), which needs to dissect each packet
> 
> and
> > compute its hash.
> > Such scenario is easily triggered. e.g. from DNS server under relevant
> 
> load, as
> > the user-space process is moved away from the CPUs serving the softirqs
> 
> (note:
> > this is beneficial for the overall DNS server performances).
> > This series introduces an helper to easily setup up XPS mapping for all
> 
> the
> > online CPUs, and use it in the ixgbe driver, demonstrating a relevant
> > performance improvement in the above scenario.
> > Paolo Abeni (2):
> >    net: introduce netif_set_xps()
> >    ixgbe: setup XPS via netif_set_xps()
> 
> 
> Resent, not HTML this time, sorry for duplication.
> 
> I truly believe XPS should not be setup by devices.
> 
> XPS is policy, and policy does belong to user space.

Thank you for your comments!

As general principle, I agree policies should be in user-space, but I
also think that the kernel should provide a reasonable default. Many MQ
devices already configure XPS and their default is AFAICS sub-optimal.

> Note that if XPS is not setup, MQ queue selection is just fine by default ;

I'm sorry, I do not follow. AFAICS with unconnected sockets without XPS
we always hit the netdev_pick_tx()/skb_tx_hash()/skb_flow_dissect()
overhead in xmit path.

Cheers,

Paolo