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

Pallipadi, Venkatesh
2005-06-08T22:14:57Z

>-----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);
+ }

Thanks,
Venki