linux-kernel - Re: [PATCH net-next] virtio-net: enable multiqueue by default

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Date:   Fri, 25 Nov 2016 13:36:42 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Hannes Frederic Sowa <hannes@...hat.com>,
        Neil Horman <nhorman@...hat.com>,
        Jeremy Eder <jeder@...hat.com>,
        Marko Myllynen <myllynen@...hat.com>,
        Maxime Coquelin <maxime.coquelin@...hat.com>
Subject: Re: [PATCH net-next] virtio-net: enable multiqueue by default



On 2016年11月25日 12:43, Michael S. Tsirkin wrote:
> On Fri, Nov 25, 2016 at 12:37:26PM +0800, Jason Wang wrote:
>> >We use single queue even if multiqueue is enabled and let admin to
>> >enable it through ethtool later. This is used to avoid possible
>> >regression (small packet TCP stream transmission). But looks like an
>> >overkill since:
>> >
>> >- single queue user can disable multiqueue when launching qemu
>> >- brings extra troubles for the management since it needs extra admin
>> >   tool in guest to enable multiqueue
>> >- multiqueue performs much better than single queue in most of the
>> >   cases
>> >
>> >So this patch enables multiqueue by default: if #queues is less than or
>> >equal to #vcpu, enable as much as queue pairs; if #queues is greater
>> >than #vcpu, enable #vcpu queue pairs.
>> >
>> >Cc: Hannes Frederic Sowa<hannes@...hat.com>
>> >Cc: Michael S. Tsirkin<mst@...hat.com>
>> >Cc: Neil Horman<nhorman@...hat.com>
>> >Cc: Jeremy Eder<jeder@...hat.com>
>> >Cc: Marko Myllynen<myllynen@...hat.com>
>> >Cc: Maxime Coquelin<maxime.coquelin@...hat.com>
>> >Signed-off-by: Jason Wang<jasowang@...hat.com>
> OK at some level but all uses of num_online_cpus()
> like this are racy versus hotplug.
> I know we already have this bug but shouldn't we fix it
> before we add more?

Not sure I get the point, do you mean adding get/put_online_cpus()? But 
is it a real bug? We don't do any cpu specific things so I believe it's 
not necessary (unless we want to keep #queues == #vcpus magically but I 
don't think so). Admin need to re-configure #queues after cpu hotplug if 
they wish.

Thanks