netdev - Re: [PATCH net-next 1/2] udp: Simplify __udp*_lib_mcast

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Date:	Tue, 15 Jul 2014 23:23:48 -0400
From:	David Held <drheld@...gle.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, Eric Dumazet <eric.dumazet@...il.com>,
	Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next 1/2] udp: Simplify __udp*_lib_mcast_deliver.

Will resubmit the patchset with that update.

On Tue, Jul 15, 2014 at 9:53 PM, David Held <drheld@...gle.com> wrote:
> On Tue, Jul 15, 2014 at 8:14 PM, David Miller <davem@...emloft.net> wrote:
>> From: David Held <drheld@...gle.com>
>> Date: Mon, 14 Jul 2014 18:30:38 -0400
>>
>>>       spin_lock(&hslot->lock);
>>> -     sk = sk_nulls_head(&hslot->head);
>>> -     dif = skb->dev->ifindex;
>>> -     sk = udp_v4_mcast_next(net, sk, uh->dest, daddr, uh->source, saddr, dif);
>>> -     while (sk) {
>>> -             stack[count++] = sk;
>>> -             sk = udp_v4_mcast_next(net, sk_nulls_next(sk), uh->dest,
>>> -                                    daddr, uh->source, saddr, dif);
>>> -             if (unlikely(count == ARRAY_SIZE(stack))) {
>>> -                     if (!sk)
>>> -                             break;
>>> -                     flush_stack(stack, count, skb, ~0);
>>> -                     count = 0;
>>> +     sk_nulls_for_each(sk, node, &hslot->head) {
>>> +             if (__udp_is_mcast_sock(net, sk,
>>> +                                     uh->dest, daddr,
>>> +                                     uh->source, saddr,
>>> +                                     dif, hnum)) {
>>> +                     stack[count++] = sk;
>>> +                     if (unlikely(count == ARRAY_SIZE(stack))) {
>>> +                             flush_stack(stack, count, skb, ~0);
>>> +                             count = 0;
>>> +                     }
>>>               }
>>>       }
>>
>> I think you are changing the logic of the loop in the edge case.
>>
>> If we have exactly ARRAY_SIZE(stack) sockets to process, the old code
>> performs the flush_stack() outside of the hslot->lock, but with your
>> change we'll do it inside the lock.
>>
>> The tradeoff here is reducing hslot->lock hold times vs. avoiding
>> taking a hold on all the sockets in the stack[] array.
>>
>> I just wanted to point this out and make sure you are aware of and
>> are ok with this.
>
> The followup patch makes it so we always take a hold on the sockets
> anyway (it makes things simpler and only changes things for the too
> large array case), so might as well reduce the lock time for the
> exactly ARRAY_SIZE case.
>
> Should just be a matter of moving the stack addition below the ARRAY_SIZE check:
> +                       if (unlikely(count == ARRAY_SIZE(stack))) {
> +                               flush_stack(stack, count, skb, ~0);
> +                               count = 0;
> +                       }
> +                       stack[count++] = sk;
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