netdev - Re: [PATCH v2] net: batch skb dequeueing from softnet input_pkt

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Date:	Tue, 13 Apr 2010 17:50:29 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH v2] net: batch skb dequeueing from softnet input_pkt_queue

On Tue, Apr 13, 2010 at 4:08 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>        Probably not necessary.
>
>> +     volatile bool           flush_processing_queue;
>
> Use of 'volatile' is strongly discouraged, I would say, forbidden.
>

volatile is used to avoid compiler optimization.

> Its usually a sign of 'I dont exactly what memory ordering I need, so I
> throw a volatile just in case'. We live in a world full of RCU, read ,
> write, full barriers. And these apis are well documented.
>

There isn't memory accessing order problem.

>> @@ -2803,6 +2808,7 @@ static void flush_backlog(void *arg)
>>                       __skb_unlink(skb, &queue->input_pkt_queue);
>>                       kfree_skb(skb);
>>               }
>> +     queue->flush_processing_queue = true;
>
>        Probably not necessary
>

If flush_backlog() is called when there are still packets in
processing_queue, there maybe some packets refer to the netdev gone,
if we remove this line.

>>       rps_unlock(queue);
>>  }
>>
>> @@ -3112,14 +3118,23 @@ static int process_backlog(struct napi_struct *napi, int quota)
>>       struct softnet_data *queue = &__get_cpu_var(softnet_data);
>>       unsigned long start_time = jiffies;
>>
>> +     if (queue->flush_processing_queue) {
>
> Really... this is bloat IMHO


Any better idea?

>
>>
>
> I advise to keep it simple.
>
> My suggestion would be to limit this patch only to process_backlog().
>
> Really if you touch other areas, there is too much risk.
>
> Perform sort of skb_queue_splice_tail_init() into a local (stack) queue,
> but the trick is to not touch input_pkt_queue.qlen, so that we dont slow
> down enqueue_to_backlog().
>
> Process at most 'quota' skbs (or jiffies limit).
>
> relock queue.
> input_pkt_queue.qlen -= number_of_handled_skbs;
>

Oh no, in order to let latter packets in as soon as possible, we have
to update qlen immediately.

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