Ingo Molnar a =E9crit :
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> * Eric Dumazet <dada1@cosmosbay.com> wrote:
>
>  =20
>> Problem is to check available space :
>>
>> It depends on stack growing UP or DOWN, and depends on caller running =

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>> on process stack, or softirq stack, or even hardirq stack.
>>    =20
>
> ok - i wish such threads were on lkml so that everyone not just the=20
> netdev kabal can read it. It's quite ugly, but if we want to check stac=
k=20
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> free space i'd suggest for you to put a stack_can_recurse() call into=20
> arch/x86/kernel/process.c and offer a default __weak implementation in =

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> kernel/fork.c that always returns 0.
>
> the rule on x86 should be something like this: on 4K stacks and 64-bit =

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> [which have irqstacks] free stack space can go as low as 25%. On 8K=20
> stacks [which doesnt have irqstacks but nests irqs] it should not go=20
> below 50% before falling back to the explicitly queued packet branch.
>
> this way other pieces of kernel code code can choose between on-stack=20
> fast recursion and explicit iterators. Although i'm not sure i like the=
=20
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> whole concept to begin with ...
>
>  =20

Hi Ingo

I took the time to prepare a patch to implement =20
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=20
netif_rx() and its slowdown if
sufficient stack space is available.

We introduce a generic arch_stack_can_recurse() which default to a weak=20
function returning 0.

 On x86 arch, we implement following logic :

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

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@cosmosbay.com>