netdev - Re: [RFC PATCH] net, rps: bypass enqueue_to

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Message-ID: <1387375442.19078.310.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 18 Dec 2013 06:04:02 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Zhi Yong Wu <zwu.kernel@...il.com>
Cc:	therbert@...gle.com, netdev@...r.kernel.org,
	Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH] net, rps: bypass enqueue_to_backlog()

On Wed, 2013-12-18 at 05:56 -0800, Eric Dumazet wrote:
> On Wed, 2013-12-18 at 21:03 +0800, Zhi Yong Wu wrote:
> > From: Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>
> > 
> > When local cpu is just target cpu which will handle network soft irq,
> > the packet should be directly injected to network stack, by bypassing
> > enqueue_to_backlog(), it can speed up the packet processing.
> > 
> > HI, guys
> > 
> > I checked the first several versions of RPS patch which seemed to have
> > this condition determination, but why was it removed later? Do i miss
> > anything? if yes, please correct me, thanks.
> > 
> > Signed-off-by: Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>
> > ---
> 
> Hmm... Could you elaborate ?
> 
> At which point do you think this condition was tested or removed ?
> 
> I think the idea was to drain NIC RX queues as fast as possible, then :
> 
> - Send the IPI to remote cpus
> - process our queue in parallel with other cpus processing their own
> queue.
> 
> If we process our packets through whole stack, packets for other cpus
> will have a fair amount of extra latency.
> 
> Thats a tradeoff I suppose.
> 

Also note that going through the backlog permits thinks like

99bbc70741903c0  ("rps: selective flow shedding during softnet
overflow")



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