netdev - Re: [PATCH net-next 2/2] net: exit busy loop when another process is runnable

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Date:	Mon, 25 Aug 2014 16:16:05 +0300
From:	Eliezer Tamir <eliezer.tamir@...ux.intel.com>
To:	Eric Dumazet <eric.dumazet@...il.com>,
	Jason Wang <jasowang@...hat.com>
CC:	Ingo Molnar <mingo@...nel.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>, davem@...emloft.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	mst@...hat.com, Peter Zijlstra <peterz@...radead.org>,
	"Ingo Molnar jacob.e.keller@...el.com" <mingo@...e.hu>
Subject: Re: [PATCH net-next 2/2] net: exit busy loop when another process
 is runnable

On 22/08/2014 17:16, Eric Dumazet wrote:
> On Fri, 2014-08-22 at 17:08 +0800, Jason Wang wrote:
> 
>> But this is just for current process. We want to determine whether or
>> not it was worth to loop busily in current process by checking if
>> there's any another runnable processes or callbacks. And what we need
>> here is just a simple and lockless hint which can't be wrong but may be
>> inaccurate to exit the busy loop. The net code does not depends on this
>> hint to do scheduling or yielding.
>>
>> How about just introducing a boolean helper like current_can_busy_loop()
>> and return true in one of the following conditions:
>>
>> - Current task is SCHED_FIFO
>> - Current task is neither SCHED_FIFO nor SCHED_IDLE and no other
>> runnable processes or pending RCU callbacks in current cpu
>>
>> And add warns to make sure it can only be called in process context.
> 
> 
> 1) Any reasons Eliezer Tamir is not included in the CC list ?

Thanks for remembering me, Eric ;)

Here are my 2 cents:
I think Ingo's suggestion of only yielding to tasks with same or higher
priority makes sense.

IF you change the current behavior, please update the documentation.
You are going to make people scratch their head and ask "what changed?"
you owe them a clue.

I also would like to have some way to keep track of when/if/how much
this yield happens.

> 2) It looks like sk_buy_loop() should not be inlined, its is already too
> big.

You made this comment in the past, my response was that it's inlnied so
it can be optimized when nonblock is known at compile time to be true
(e.g when called from sock_poll).
IFF you think that's less important, then I will defer to your opinion.

-Eliezer
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