linux-kernel - Re: [PATCH 1/2] Avoid bio

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Date:	Thu, 3 Jul 2008 17:03:44 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
cc:	Jens Axboe <jens.axboe@...cle.com>, linux-kernel@...r.kernel.org,
	Neil Brown <neilb@...e.de>
Subject: Re: [PATCH 1/2] Avoid bio_endio recursion

On Wed, 2 Jul 2008, Alan Cox wrote:

> On Wed, 2 Jul 2008 00:09:22 -0400 (EDT)
> Mikulas Patocka <mpatocka@...hat.com> wrote:
>
>>> Right, that wont work of course. Completions are typically done through
>>> a softirq, so it is not currently done with hard interrupts disabled.
>>
>> I thought, from hardirq - that's what IDE is doing. And they are called
>
> Even IDE will sometimes complete from a timer on an error.
>
>> And does local_irq_restore() need to execute even more costy "popf" when
>> it makes a transition from disabled to disabled state? What's
>> local_irq_restore semantics --- is it allowed to use local_irq_restore for
>> transition from interrupt-enabled state into interrupt-disabled state?
>
> If you are worried about performance the network layer has _irq variants
> of various functions that are faster and can only be called from the
> right context (eg kfree_skb_irq), so you could do two versions of that
> code.
>
> Alan

I was thinking about a general optimization --- something like this. On 
Core2 it doesn't make much sense --- cli is fast there (just 11 ticks). On 
Pentium 4 it would make sense, because cli is much more costy there.

Mikulas

---
  include/asm-x86/irqflags.h |    3 ++-
  1 file changed, 2 insertions(+), 1 deletion(-)

Index: linux-2.6.26-rc8/include/asm-x86/irqflags.h
===================================================================
--- linux-2.6.26-rc8.orig/include/asm-x86/irqflags.h	2008-06-02 23:13:32.000000000 +0200
+++ linux-2.6.26-rc8/include/asm-x86/irqflags.h	2008-07-03 22:53:32.000000000 +0200
@@ -101,7 +101,8 @@ static inline unsigned long __raw_local_
  {
  	unsigned long flags = __raw_local_save_flags();

-	raw_local_irq_disable();
+	if (flags & X86_EFLAGS_IF)
+		raw_local_irq_disable();

  	return flags;
  }
--
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