linux-kernel - Re: [patch 1/6] hardirq: Make hardirq bits generic

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Date:	Thu, 7 Nov 2013 15:12:23 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Andreas Schwab <schwab@...ux-m68k.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <ak@...ux.intel.com>, Peter Anvin <hpa@...or.com>,
	Mike Galbraith <bitbucket@...ine.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Linux/m68k" <linux-m68k@...r.kernel.org>
Subject: Re: [patch 1/6] hardirq: Make hardirq bits generic

Hi Thomas,

On Wed, Nov 6, 2013 at 6:23 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>> Also note that the value of "nested" doesn't match the indentation level,
>> which depends on my own bookkeeping using "nesting".
>
> Well, nested is just an indicator. It's not the nest level.

I know, the only thing that matters is whether it's zero or not.
But it should always be zero if there's no nesting, and non-zero if there
is, right?

So:

#   irq 13 nested 1024

nested should be 0 here.

#   irq 4 nested 0

ok

#     irq 13 nested 1024

ok (two extra spaces in front of "irq").

#     irq 4 nested 0

nested should be non-zero here.

>       nested = pt->sr & ~ALLOWINT;
> i.e.:
>       nested = pt->sr & 0x0700;
>
> So in the case above nested is 0x400
>
>> Anyone with an idea where it's going wrong?
>
> The original code does:
>
>     add_preempt_count(HARDIRQ_OFFSET);
>
>     do_IRQ()
>         irq_enter();
>           add_preempt_count(HARDIRQ_OFFSET);
>
>         handle_irq();
>
>         irq_exit();
>             local_irq_disable();
>             sub_preempt_count(HARDIRQ_OFFSET);
>
>     sub_preempt_count(HARDIRQ_OFFSET);
>
>     /* Check for nested irq */
>     if (in_hardirq())
>        reti();
>
>     /* Check for nested irq again */
>     if (pt->sr & ~ALLOWINT != 0)
>        reti();
>
>     do_softirq();
>        ....
>     ret_from_exception();
>
> With the patches in place it looks like this:
>
>      do_IRQ()
>         nested = pt->sr & ~ALLOWINT;
>
>         irq_enter();
>           add_preempt_count(HARDIRQ_OFFSET);
>
>         handle_irq();
>
>         irq_exit_nested(nested);
>             local_irq_disable();
>             sub_preempt_count(HARDIRQ_OFFSET);
>             if (!nested && !in_hardirq())
>                do_softirq()
>
>         return nested;
>
>       if (nested)
>          reti();
>
>       ret_from_exception();
>
> So all it does essentially is to move the softirq invocation in the
> non nested case a tad earlier. I'm really puzzled as I can't spot the
> point where this change makes a real difference.

Yes, that's also my understanding.
But I can't spot it neither :-(

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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