linux-kernel - Re: [RFC,PATCH] loopback: calls netif_receive_skb() instead of netif

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Date:	Tue, 01 Apr 2008 11:19:06 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFC,PATCH] loopback: calls netif_receive_skb() instead of	netif_rx()

Ingo Molnar a écrit :
> * Eric Dumazet <dada1@...mosbay.com> wrote:
>
>   
>> Problem is to check available space :
>>
>> It depends on stack growing UP or DOWN, and depends on caller running 
>> on process stack, or softirq stack, or even hardirq stack.
>>     
>
> ok - i wish such threads were on lkml so that everyone not just the 
> netdev kabal can read it. It's quite ugly, but if we want to check stack 
> free space i'd suggest for you to put a stack_can_recurse() call into 
> arch/x86/kernel/process.c and offer a default __weak implementation in 
> kernel/fork.c that always returns 0.
>
> the rule on x86 should be something like this: on 4K stacks and 64-bit 
> [which have irqstacks] free stack space can go as low as 25%. On 8K 
> stacks [which doesnt have irqstacks but nests irqs] it should not go 
> below 50% before falling back to the explicitly queued packet branch.
>
> this way other pieces of kernel code code can choose between on-stack 
> fast recursion and explicit iterators. Although i'm not sure i like the 
> whole concept to begin with ...
>
>   

Hi Ingo

I took the time to prepare a patch to implement  
arch_stack_can_recurse() as you suggested.

Thank you

[PATCH] x86 : arch_stack_can_recurse() introduction

Some paths in kernel would like to chose between on-stack fast recursion 
and explicit iterators.

One identified spot is in net loopback driver, where we can avoid 
netif_rx() and its slowdown if
sufficient stack space is available.

We introduce a generic arch_stack_can_recurse() which default to a weak 
function returning 0.

 On x86 arch, we implement following logic :

   32 bits and 4K stacks (separate irq stacks) : can use up to 25% of stack
   64 bits, 8K stacks (separate irq stacks)    : can use up to 25% of stack
   32 bits and 8K stacks (no irq stacks)       : can use up to 50% of stack

Example of use in drivers/net/loopback.c, function  loopback_xmit()

if (arch_stack_can_recurse())
    netif_receive_skb(skb); /* immediate delivery to stack */
else
    netif_rx(skb); /* defer to softirq handling */

Signed-off-by: Eric Dumazet <dada1@...mosbay.com>



View attachment "can_recurse.patch" of type "text/plain" (2283 bytes)