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

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Message-Id: <20100422.002118.107274505.davem@davemloft.net>
Date:	Thu, 22 Apr 2010 00:21:18 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
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()

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 :-)
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