netdev - Re: [PATCH net-next 2/2] bnx2x: use the default NAPI weight

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Date:	Wed, 06 Mar 2013 14:59:47 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, eilong@...adcom.com, jhs@...atatu.com,
	herbert@...dor.apana.org.au
Subject: Re: [PATCH net-next 2/2] bnx2x: use the default NAPI weight

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 05 Mar 2013 23:03:18 -0800

> On Tue, 2013-03-05 at 23:37 -0500, David Miller wrote:
> 
>> Thanks for the explanation.
>> 
>> Since you haven't completely resolved the issues you were running into
>> I'll target this to net-next for now.
> 
> Thanks David
> 
> An other issue is the spin_trylock() attempted in net_tx_action()
> 
> It seems we can miss a qdisc_run(), and have to wait the following
> NET_TX softirq(s) to send more data. NET_RX being interleaved, we can
> have to wait a long time (not mentioning other softirq handlers like
> RCU ...)
> 
> I might be too tired right now, but cant see the reason of the trylock.
> 
> qdisc lock is already BH safe, so we should do a spinlock
 ...
> @@ -3201,22 +3201,11 @@ static void net_tx_action(struct softirq_action *h)
>  			head = head->next_sched;
>  
>  			root_lock = qdisc_lock(q);
> -			if (spin_trylock(root_lock)) {
> -				smp_mb__before_clear_bit();
> -				clear_bit(__QDISC_STATE_SCHED,
> -					  &q->state);
> -				qdisc_run(q);
> -				spin_unlock(root_lock);

I think this trylock is intentional, but not to deal with BH safeness,
but rather to allow another cpu already processing the qdisc to
continue doing so.

I think this is what Jamal's amazing flash animations back at netconf
in Toronto were all about :-)

Herbert Xu and Jamal have touched upon this issue several times in
the past.
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