netdev - Re: [PATCH v5] net: batch skb dequeueing from softnet input_pkt

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Date:	Thu, 22 Apr 2010 16:33:25 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>, jamal <hadi@...erus.ca>,
	Tom Herbert <therbert@...gle.com>, netdev@...r.kernel.org
Subject: Re: [PATCH v5] net: batch skb dequeueing from softnet
 input_pkt_queue

Le jeudi 22 avril 2010 à 21:06 +0800, Changli Gao a écrit :
> On Thu, Apr 22, 2010 at 7:37 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> >
> > Please reorder things better.
> >
> > Most likely this function is called for one packet.
> >
> > In your version you take twice the rps_lock()/rps_unlock() path, so
> > it'll be slower.
> >
> > Once to 'transfert' one list to process list
> >
> > Once to be able to do the 'label out:' post processing.
> >
> 
> It doesn't make any difference. We have to hold rps_lock to update
> input_pkt_queue_len, if we don't use another variable to record the
> length of the process queue, or atomic variable.
> 

It does make a difference, Damn it.

I really really start to think you dont read what I wrote, or you dont
care.

Damn, cant you update all the things at once, taking this lock only
once ?

You focus having an ultra precise count of pkt_queue.len, but we dont
care at all ! We only want a _limit_, or else the box can be killed by
DOS.

If in practice this limit can be 2*limit, thats OK. 

Cant you understand this ?

> I think it is better that using another variable to record the length
> of the process queue, and updating it before process_backlog()
> returns. For one packet, there is only one locking/unlocking. There is
> only one issue you concerned: cache miss due to sum the two queues'
> length. I'll change softnet_data to:
> 
> struct softnet_data {
>         struct Qdisc            *output_queue;
>         struct list_head        poll_list;
>         struct sk_buff          *completion_queue;
>         struct sk_buff_head     process_queue;
> 
> #ifdef CONFIG_RPS
>         struct softnet_data     *rps_ipi_list;
> 
>         /* Elements below can be accessed between CPUs for RPS */
>         struct call_single_data csd ____cacheline_aligned_in_smp;
>         struct softnet_data     *rps_ipi_next;
>         unsigned int            cpu;
>         unsigned int            input_queue_head;
> #endif
>         unsigned int            process_queue_len;
>         struct sk_buff_head     input_pkt_queue;
>         struct napi_struct      backlog;
> };
> 
> For one packets, we have to update process_queue_len in any way. For
> more packets, we only change process_queue_len just before
> process_backlog() returns. It means that process_queue_len change is
> batched.
> 

We need one limit. Not two limits.

I already told you how to do it, but you ignored me and started yet
another convoluted thing.


process_backlog() transfert the queue to its own queue and reset pkt_len
to 0 (Only once)

End of story.

Maximum packet queued to this cpu softnet_data will be 2 * old_limit.

So what ?


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