Hi Thomas,
On Mon, Nov 11, 2013 at 9:52 PM, Thomas Gleixner <t...@linutronix.de> wrote:
> Some more thoughts on this.
>
> The whole nesting check in the exisiting low level entry code and what
> I tried to resemble with the irq_exit_nested() is pretty pointless.
>
> Let's look at auto_inthandler and ret_from_exception
>
> ENTRY(auto_inthandler)
> SAVE_ALL_INT
> GET_CURRENT(%d0)
> movel %d0,%a1
> addqb #1,%a1@(TINFO_PREEMPT+1)
> | put exception # in d0
> bfextu %sp@(PT_OFF_FORMATVEC){#4,#10},%d0
> subw #VEC_SPUR,%d0
>
> movel %sp,%sp@-
> movel %d0,%sp@- | put vector # on stack
> auto_irqhandler_fixup = . + 2
> jsr do_IRQ | process the IRQ
> addql #8,%sp | pop parameters off stack
>
> ret_from_interrupt:
> movel %curptr@(TASK_STACK),%a1
> subqb #1,%a1@(TINFO_PREEMPT+1)
> jeq ret_from_last_interrupt
> 2: RESTORE_ALL
>
> ALIGN
> ret_from_last_interrupt:
> moveq #(~ALLOWINT>>8)&0xff,%d0
> andb %sp@(PT_OFF_SR),%d0
> jne 2b
>
> /* check if we need to do software interrupts */
> tstl irq_stat+CPUSTAT_SOFTIRQ_PENDING
> jeq .Lret_from_exception
> pea ret_from_exception
> jra do_softirq
>
>
> ENTRY(ret_from_exception)
> .Lret_from_exception:
> btst #5,%sp@(PT_OFF_SR) | check if returning to kernel
> bnes 1f | if so, skip resched, signals
> ....
> 1: RESTORE_ALL
>
> So in every interrupt exit path we check:
>
> 1) Whether the hardirq part of preempt_count is zero
>
> 2) Whether the interrupt prio mask of SR on stack is zero
>
> and if we finally reach ret_from_exception we have the final check:
>
> 3) whether we return to kernel or user space.
>
> And this final check is the only one which matters, really.
>
> If you look at the probability of the first two checks catching
> anything, then it's pretty low. Most interrupts returns go through
> ret_from_exception. Yes, I added counters which prove that at least on
> the aranym, but I doubt that it will make a real difference if you run
> this on real hardware.
>
> So what's the point of having these checks in the hotpath? The patch
Most of this seems to be as old as stone-age. It was rewritten for
v2.5.29, but the initial bookkeeping was there in v2.1, and even in some
form in v1.3.94.
> below against 3.12 vanilla works nicely on the aranym and I don't see
> a reason why this extra hackery is necessary at all. It's just code
> bloat in a hotpath.
>
> Now the only valid concern might be do_softirq itself, but that's
> pointless as well. If the softirq is interrupted, then we do not
> invoke it again. If the nested interrupt happens before irq_exit()
> actually disables interrupts, then we won't invoke it either as the
> hardirq part of preempt_count is still not zero.
>
> As a side note: The do_softirq calls in the platform/68xxx entry
> pathes are just copied leftovers as well. Both entry code pathes are
> not fiddling with the preempt count and both call do_IRQ() which will
> call irq_exit() at the end which will invoke do_softirq(), so the
> check for more softirqs in the irq return path is just pointless.
Your reasoning sounds OK, and it works on ARAnyM, so, thanks again and
Acked-by: Geert Uytterhoeven <ge...@linux-m68k.org>
BTW, do you plan to get the "make hardirq bits generic" series in 3.13?
Or do you want me to take this patch through the m68k tree?
So far I don't have any plans to send another pull request for 3.13.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@linux-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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