On Fri, May 09, 2014 at 05:47:12PM +1000, Anton Blanchard wrote:
> I am seeing an issue where a CPU running perf eventually hangs.
> Traces show timer interrupts happening every 4 seconds even
> when a userspace task is running on the CPU.
Is this by chance every 4.2 seconds? The reason I ask is that
Paul Clarke and I are seeing an interrupt every 4.2 seconds when
he runs NO_HZ_FULL, and are trying to get rid of it. ;-)
Thanx, Paul
> /proc/timer_list
> also shows pending hrtimers have not run in over an hour,
> including the scheduler.
>
> Looking closer, decrementers_next_tb is getting set to
> 0xffffffffffffffff, and at that point we will never take
> a timer interrupt again.
>
> In __timer_interrupt() we set decrementers_next_tb to
> 0xffffffffffffffff and rely on ->event_handler to update it:
>
> *next_tb = ~(u64)0;
> if (evt->event_handler)
> evt->event_handler(evt);
>
> In this case ->event_handler is hrtimer_interrupt. This will eventually
> call back through the clockevents code with the next event to be
> programmed:
>
> static int decrementer_set_next_event(unsigned long evt,
> struct clock_event_device *dev)
> {
> /* Don't adjust the decrementer if some irq work is pending */
> if (test_irq_work_pending())
> return 0;
> __get_cpu_var(decrementers_next_tb) = get_tb_or_rtc() + evt;
>
> If irq work came in between these two points, we will return
> before updating decrementers_next_tb and we never process a timer
> interrupt again.
>
> This looks to have been introduced by 0215f7d8c53f (powerpc: Fix races
> with irq_work). Fix it by removing the early exit and relying on
> code later on in the function to force an early decrementer:
>
> /* We may have raced with new irq work */
> if (test_irq_work_pending())
> set_dec(1);
>
> Signed-off-by: Anton Blanchard <an...@samba.org>
> Cc: sta...@vger.kernel.org # 3.14+
> ---
>
> diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
> index 122a580..4f0b676 100644
> --- a/arch/powerpc/kernel/time.c
> +++ b/arch/powerpc/kernel/time.c
> @@ -813,9 +888,6 @@ static void __init clocksource_init(void)
> static int decrementer_set_next_event(unsigned long evt,
> struct clock_event_device *dev)
> {
> - /* Don't adjust the decrementer if some irq work is pending */
> - if (test_irq_work_pending())
> - return 0;
> __get_cpu_var(decrementers_next_tb) = get_tb_or_rtc() + evt;
> set_dec(evt);
>
>
_______________________________________________
Linuxppc-dev mailing list
Linuxppc-dev@lists.ozlabs.org
https://lists.ozlabs.org/listinfo/linuxppc-dev
