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

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Message-ID: <CAEH94Lh-Dujb9YbpFK0kTu2O3pGSf16rXwCWFWBE50K_4geHnA@mail.gmail.com>
Date:	Wed, 18 Dec 2013 22:11:03 +0800
From:	Zhi Yong Wu <zwu.kernel@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Tom Herbert <therbert@...gle.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH] net, rps: bypass enqueue_to_backlog()

On Wed, Dec 18, 2013 at 10:04 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> 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"
it makes sense, but if cpu < 0, some packets seems to bypass the flow
shedding stuff, That is, the flow limit will be not so accurate,
right?

>
>
>



-- 
Regards,

Zhi Yong Wu
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