netdev - Re: [PATCH] RPS: support 802.1q and pppoe session

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Date:	Thu, 25 Mar 2010 13:47:48 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Tom Herbert <therbert@...gle.com>, netdev@...r.kernel.org
Subject: Re: [PATCH] RPS: support 802.1q and pppoe session

On Thu, Mar 25, 2010 at 1:24 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le jeudi 25 mars 2010 à 13:12 +0800, Changli Gao a écrit :
>>
>> It is useful when Linux is run as a bridge.
>>
>
> I am not saying its not useful. BTW, for routers/bridges, RPS is not a
> good idea, unless you must add netfilter complex rules.
>

Yea, we do DPI in netfilter. And for a stateful fireware, connection
tracking isn't cheap. As bandwidth increases, we find one CPU can't
handle all the traffic from a single NIC. We currently use dynamic
weighted packets distributing algorithm with patched Linux-2.6.18, and
it works very well.

>> >
>> > I agree we might need a flag or something to reset rxhash to 0 somewhere
>> > (probably in non accelerated vlan rx handling) to force second
>> > get_rps_cpu() invocation to recompute it. This small correction has no
>> > cost if put outside of get_rps_cpus().
>> >
>> > If get_rps_cpus() is too complex, it might become too slow for typical
>> > use. We should find smart ways to solve your performance problem if they
>> > ever exist.
>> >
>>
>> It means that more than one IPI will be sent for just a single
>> packets, I don't think the cost is acceptable.
>>
>
> I believe you dont _fully_ understand how RPS currently works.
>
> I am very surprised you send RPS patches if you dont master it.
>
> Please read again get_rps_cpus(), line 2238
>
>        default:
>                goto done;   <<<< HERE skb->rxhash unchanged >>>>
>        }
>        ports = 0;
>        switch (ip_proto) {
>
>
> This means that unknown protocols are directly handled by THIS cpu, and
> not given to another cpu. No IPI involved.
>
> In case of tunnels or vlan, we then reenter lowlevel stack and at this
> point, we can fully use RPS, because we are able to find IPV4/IPV6
> headers.
>
> Your patch is not necessary, since next time we call get_rps_cpus(),
> rxhash being still null, we compute the correct non null hash and at
> this point can choose an appropriate target cpu.
>
> (All you need is to set /sys/class/net/vlan.825/queues/rx-0/rps_cpus to
> needed mask)
>

I knew the current code is OK, and no additional IPI is needed. I said
that because you said:

>> > I agree we might need a flag or something to reset rxhash to 0 somewhere
>> > (probably in non accelerated vlan rx handling) to force second
>> > get_rps_cpu() invocation to recompute it. This small correction has no
>> > cost if put outside of get_rps_cpus().

Oh, maybe I misunderstood you words. I thought the rxhash you want to
clear is computed by get_rps_cpu()? I remembered some NIC itself can
get 5-tuple from vlan and pppoe packets to compute hash. In that case,
we should not clear rxhash.

-- 
Regards，
Changli Gao(xiaosuo@...il.com)
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