netdev - Re: [PATCH net-next-2.6] net: Xmit Packet Steering (XPS)

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Date:	Fri, 20 Nov 2009 13:11:39 +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>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next-2.6] net: Xmit Packet Steering (XPS)

On Fri, Nov 20, 2009 at 12:58 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Changli Gao a écrit :
>> On Fri, Nov 20, 2009 at 7:46 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>>> diff --git a/net/core/dev.c b/net/core/dev.c
>>> index 9977288..9e134f6 100644
>>> --- a/net/core/dev.c
>>> +++ b/net/core/dev.c
>>> @@ -2000,6 +2001,7 @@ gso:
>>>         */
>>>        rcu_read_lock_bh();
>>>
>>> +       skb->sending_cpu = cpu = smp_processor_id();
>>>        txq = dev_pick_tx(dev, skb);
>>>        q = rcu_dereference(txq->qdisc);
>>
>> I think assigning cpu to skb->sending_cpu just before calling
>> hard_start_xmit is better, because the CPU which dequeues the skb will
>> be another one.
>
> I want to record the application CPU, because I want the application CPU
> to call sock_wfree(), not the CPU that happened to dequeue skb to transmit it
> in case of txq contention.
>

got it.

>>
>>> @@ -2024,8 +2026,6 @@ gso:
>>>           Either shot noqueue qdisc, it is even simpler 8)
>>>         */
>>>        if (dev->flags & IFF_UP) {
>>> -               int cpu = smp_processor_id(); /* ok because BHs are off */
>>> -
>>>                if (txq->xmit_lock_owner != cpu) {
>>>
>>>                        HARD_TX_LOCK(dev, txq, cpu);
>>> @@ -2967,7 +2967,7 @@ static void net_rx_action(struct softirq_action *h)
>>>        }
>>>  out:
>>>        local_irq_enable();
>>> -
>>> +       xps_flush();
>>
>> If there isn't any new skbs, the memory will be hold forever. I know
>> you want to eliminate unnecessary IPI, how about sending IPI only when
>> the remote xps_pcpu_queues are changed from empty to nonempty?
>
> I dont understand your remark, and dont see the problem, yet.
>
> I send IPI only on cpus I know I have at least one skb queueud for them.
> For each cpu taking TX completion interrupts I have :
>
> One bitmask (xps_cpus) of cpus I will eventually send IPI at end of net_rx_action()
>

You call xps_flush() in net_rx_aciton(). It means that if no new
packet arrives, xps_flush() won't be called forever, and the memory
used by skbs will be hold forever. Did I misunderstand? Your algorithm
only works with packet forwarding but sending packets from local
sockets.

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