netdev - Re: [PATCH net-next-2.6] rps: immediate send IPI in process

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Date:	Thu, 22 Apr 2010 09:28:23 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	therbert@...gle.com, xiaosuo@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next-2.6] rps: immediate send IPI in
 process_backlog()

Le jeudi 22 avril 2010 à 00:21 -0700, David Miller a écrit :
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Wed, 21 Apr 2010 23:04:58 +0200
> 
> > If some skb are queued to our backlog, we are delaying IPI sending at
> > the end of net_rx_action(), increasing latencies. This defeats the
> > queueing, since we want to quickly dispatch packets to the pool of
> > worker cpus, then eventually deeply process our packets.
> > 
> > It's better to send IPI before processing our packets in upper layers,
> > from process_backlog().
> > 
> > Change the _and_disable_irq suffix to _and_enable_irq(), since we enable
> > local irq in net_rps_action(), sorry for the confusion.
> > 
> > Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> 
> Eric, irqs are enabled in process_backlog(), so I don't know how legal
> it is to invoke net_rps_action_and_irq_enable() from there.
> 
> At least, if you are depending upon a later action to pick up the
> pieces if the rps_ipi_list test races, you need to update the comment
> above net_rps_action_and_irq_enable() since it states that it is
> always invoked with IRQs disabled :-)
> --

But I do disable irqs berfore calling this function from
process_backlog, only if current pointer is non null.

Pointer is then re-fetched inside net_rps_action_and_irq_enable()

I thought using xchg(), but this adds an atomic op, so I think its
better to use local_irq_disable()/enable() pairs.


About the comment, it says :

/*
 * net_rps_action sends any pending IPI's for rps.
 * Note: called with local irq disabled, but exits with local irq
enabled.
 */


So it documents this function is called with irq disabled, and re-enable
them before return ?


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