netdev - Re: [PATCH] ifb: add multi-queue support

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Date:	Mon, 16 Nov 2009 16:43:46 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Stephen Hemminger <shemminger@...tta.com>,
	Patrick McHardy <kaber@...sh.net>,
	Tom Herbert <therbert@...gle.com>, netdev@...r.kernel.org
Subject: Re: [PATCH] ifb: add multi-queue support

2009/11/16 Eric Dumazet <eric.dumazet@...il.com>:
> Changli Gao a écrit :
>> ifb: add multi-queue support.
>>
>> Add multi-queue support, through more than one tasklets. One tasklet per
>> queue always steers on the same CPU, and if the number of the
>> tasklets(queues) is lager than the number of CPUs, more than one
>> tasklets will be assigned to the same CPU.
>>
>> Signed-off-by: Changli Gao <xiaosuo@...il.com>
>> ----
>
>>
>
> I must say I fail to see how multiple tasklets per cpu can be of any use.
>
> And :
>
> +       if (skb_queue_len(&pq->rq) == 1)
> +               tasklet_schedule_on(&pq->task, num % num_online_cpus());
>
> Is probably not what you want, because :
>
> A) num_online_cpus() can be expensive to compute,
> B) and you can have such configs :
>
> Three online cpus, CPU0 and CPU4, CPU5
> -> you call tasklet_schedule_on(... , num = {0|1|2})
>
> To avoid packets reorder, if your hash function selects an offline cpu,
> you must forward the packet to a particular cpu, regardless of cpu currently running.
> (even if real device is not multiqueue, its RX interrupts can be handled by any online cpu)
>
> You maybe need to maintain a mapping table with cpu hotplug notifiers.
>

Yea, a mapping table is needed in any way. But I don't find a suitable
user interface to do that. And I think many people maybe interested in
the two new APIs: tasklet_schedule_on() and tasklet_hi_schedule_on().
Any comment?

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