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

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Message-Id: <20161128.112823.2263657283777032168.davem@davemloft.net>
Date:   Mon, 28 Nov 2016 11:28:23 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     mst@...hat.com
Cc:     jasowang@...hat.com, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        hannes@...hat.com, nhorman@...hat.com, jeder@...hat.com,
        myllynen@...hat.com, maxime.coquelin@...hat.com
Subject: Re: [PATCH net-next] virtio-net: enable multiqueue by default

From: "Michael S. Tsirkin" <mst@...hat.com>
Date: Fri, 25 Nov 2016 06:43:08 +0200

> 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?

This is more being used like a heuristic in this scenerio, and in
fact I would say one would keep the code this way even once proper
hotplug handlers are installed to adjust the queued dynamically if
there is a desired (which is also not necessarily the case).

I really don't think this change should be held on up on this issue.
So can we please make some forward progress here?

Thanks.