On Thu, 7 Nov 2013, Geert Uytterhoeven wrote:
> Hi Thomas,
>
> On Wed, Nov 6, 2013 at 6:23 PM, Thomas Gleixner <[email protected]> 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.
Hmm. The softirq code reenables interrupts unconditionally. So when an
interrupt hits there SR on stack has the bits cleared. You could
verify that by checking in_serving_softirq() at the entry to
do_IRQ(). That could also explain the irq 4 nests in irq 4 issue. You
can't observe that on the original code as the softirq invocation and
therefor the interrupt enable happens outside of do_IRQ().
Though that does not explain the non nested case where nested is !=
0. But it looks like that irq 13 has a higher level than 4:
> # irq 13 nested 1024
>
> ok (two extra spaces in front of "irq").
So it could actually be the following:
irq X arrives, SR I2/1/0 is set to 4
Now before we reach do_IRQ()
irq 13 arrives and interrupts irq X as it has a higher level
Your nest accounting shows 0, but the SR says nested, which is
actually the correct state.
Is there an easy to setup/use emulator around on which I could try to
dig into that myself?
Thanks,
tglx
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