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

From: Pallipadi, Venkatesh (venkatesh.pallipadi_at_intel.com)
Date: 06/09/05

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    Date:	Wed, 8 Jun 2005 15:14:39 -0700
To: "Jonathan Corbet" <corbet@lwn.net>, "Tony Lindgren" <tony@atomide.com>

    >-----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
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