netdev - Re: root_lock vs. device's TX lock

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Date:	Thu, 17 Nov 2011 19:19:58 +0100
From:	Dave Taht <dave.taht@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Tom Herbert <therbert@...gle.com>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: root_lock vs. device's TX lock

On Thu, Nov 17, 2011 at 6:26 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le jeudi 17 novembre 2011 à 17:34 +0100, Eric Dumazet a écrit :
>> Le jeudi 17 novembre 2011 à 08:10 -0800, Tom Herbert a écrit :
>> > From sch_direct_xmit:
>> >
>> >         /* And release qdisc */
>> >         spin_unlock(root_lock);
>> >
>> >         HARD_TX_LOCK(dev, txq, smp_processor_id());
>> >         if (!netif_tx_queue_frozen_or_stopped(txq))
>> >                 ret = dev_hard_start_xmit(skb, dev, txq);
>> >
>> >         HARD_TX_UNLOCK(dev, txq);
>> >
>> >         spin_lock(root_lock);
>> >
>> > This is a lot of lock manipulation to basically switch from one lock
>> > to another and possibly thrashing just to send a packet.  I am
>> > thinking that if the there is a 1-1 correspondence between qdisc and
>> > device queue then we could actually use the device's lock as the root
>> > lock for the qdisc.  So in that case, we would need to touch any locks
>> > from sch_direct_xmit (just hold root lock which is already device lock
>> > for the duration).
>> >
>> > Is there any reason why this couldn't work?
>>
>> But we have to dirty part of Qdisc anyway ?
>> (state, bstats, q, ...)
>>
>
> Also we want to permit other cpus to enqueue packets to Qdisc while our
> cpu is busy in network driver ndo_start_xmit()
>
> For complex Qdisc / tc setups (potentially touching a lot of cache
> lines), we could eventually add a small ring buffer so that the cpu
> doing the ndo_start_xmit() also queues the packets into Qdisc.
>
> This ringbuffer could use a lockless algo. (we currently use the
> secondary 'busylock' to serialize other cpus, but each cpu calls qdisc
> enqueue itself.)

I was thinking ringbuffering might also help in adding a 'grouper'
abstraction to the dequeuing side.
>
>
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