Re: [PATCH] Dynamic tick for x86 version 050602-1

Tony Lindgren
2005-06-09T01:49:43Z

* Pallipadi, Venkatesh <venkatesh.pallipadi@intel.com> [050608 15:14]:
>
> >-----Original Message-----
> >From: linux-kernel-owner@vger.kernel.org
> >[mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of
> >Jonathan Corbet
> >Sent: Tuesday, June 07, 2005 1:36 PM
> >To: Tony Lindgren
> >Cc: linux-kernel@vger.kernel.org
> >Subject: Re: [PATCH] Dynamic tick for x86 version 050602-1
> >
> >Tony Lindgren <tony@atomide.com> wrote:
> >
> >> --- linux-dev.orig/arch/i386/kernel/irq.c 2005-06-01
> >17:51:36.000000000 -0700
> >> +++ linux-dev/arch/i386/kernel/irq.c 2005-06-01
> >17:54:32.000000000 -0700
> >> [...]
> >> @@ -102,6 +103,12 @@ fastcall unsigned int do_IRQ(struct pt_r
> >> );
> >> } else
> >> #endif
> >> +
> >> +#ifdef CONFIG_NO_IDLE_HZ
> >> + if (dyn_tick->state & (DYN_TICK_ENABLED |
> >DYN_TICK_SKIPPING) && irq != 0)
> >> + dyn_tick->interrupt(irq, NULL, regs);
> >> +#endif
> >> +
> >> __do_IRQ(irq, regs);
> >
> >Forgive me if I'm being obtuse (again...), but this hunk doesn't look
> >like it would work well in the 4K stacks case. When 4K stacks
> >are being
> >used, dyn_tick->interrupt() will only get called in the nested
> >interrupt
> >case, when the interrupt stack is already in use. This change also
> >pushes the non-assembly __do_IRQ() call out of the else branch, meaning
> >that, when the switch is made to the interrupt stack (most of
> >the time),
> >__do_IRQ() will be called twice for the same interrupt.
> >
> >It looks to me like you want to put your #ifdef chunk *after* the call
> >to __do_IRQ(), unless you have some reason for needing it to happen
> >before the regular interrupt handler is invoked.
> >
>
> Good catch. This indeed looks like a bug.
> With 050602-1 version I am seeing double the number of calls to
> timer_interrupt routine than expected. Say, when all CPUs are fully
> busy,
> I see 2*HZ timer interrupt count in /proc/interrupts
>
> And things look normal once I change this hunk as below
>
> >> } else
> >> #endif
> >> +
> + {
> >> +#ifdef CONFIG_NO_IDLE_HZ
> >> + if (dyn_tick->state & (DYN_TICK_ENABLED |
> >DYN_TICK_SKIPPING) && irq != 0)
> >> + dyn_tick->interrupt(irq, NULL, regs);
> >> +#endif
> >> +
> >> __do_IRQ(irq, regs);
> + }

Cool. Sorry for not responding earlier, my hard drive crashed yesterday
morning... I also managed to fry my spare computer's motherboard
while trying to recover some data from the broken disk :)

I'll try to post an updated patch tomorrow.

Tony