netdev - Re: [PATCH net-next] net: rps: don't skip offline cpus when set rps

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Date:   Mon, 15 May 2017 10:23:30 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     zlpnobody@....com
Cc:     netdev@...r.kernel.org, therbert@...gle.com, zlpnobody@...il.com
Subject: Re: [PATCH net-next] net: rps: don't skip offline cpus when set
 rps_cpus

From: Liping Zhang <zlpnobody@....com>
Date: Mon, 15 May 2017 20:55:52 +0800

> From: Liping Zhang <zlpnobody@...il.com>
> 
> On our 4-core system, sometimes I can enable all CPUs to process packets.
> But sometimes I can't, if all the CPUs become offline except core 0, I
> will get the following result, which is really annoying for my script:
>  # echo f > /sys/class/net/eth0/queues/rx-0/rps_cpus
>  # cat /sys/class/net/eth0/queues/rx-0/rps_cpus
>  1
> 
> Since we won't steer the packets to these offline cpus, it's reasonable
> to enable all configed cpus to the rps_map, even if they are offline for
> the time being.
> 
> Signed-off-by: Liping Zhang <zlpnobody@...il.com>

But this means while those cpus are offline, get_rps_cpu() performs
very suboptimally.

In fact, if the packet hashes to one of those offline cpus, you get
no RPS at all.

So I think we really can't make this change, at least in it's current
form.