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

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Date:	Thu, 17 Nov 2011 17:34:58 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: root_lock vs. device's TX lock

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, ...)


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