On Mon, Jul 28, 2014 at 07:37:33PM +0200, Frederic Weisbecker wrote:
> The timekeeper gets initialized to the value of the CPU where the
> first clockevent device is setup. This works well because the timekeeper
> can be any online CPU in most configs.
>
> Full dynticks has its own requirement though and needs the timekeeper
> to always be 0. And this requirement seem to accomodate pretty well with
> the above described boot timekeeper setting because the first clockevent
> device happens to be initialized, most of the time, on the boot CPU
> (which should be CPU 0).
This isn't true in general, Voyager (which we dropped support for iirc)
had a boot cpu != 0, and I think there's ARM platforms where the same
can be true.
> However there is no mention of such a guarantee anywhere. This assumption
> might well be defeated on some corner case now or in the future.
Right..
> So lets wipe out the FUD and force tick_do_timer_cpu to CPU 0 on boot
> when full dynticks is used.
>
> This way we can even remove some corner case code that handled scenarios
> where all clockevent devices were setup on full dynticks CPUs.
> diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
> index 0a0608e..cb57bce 100644
> --- a/kernel/time/tick-common.c
> +++ b/kernel/time/tick-common.c
> @@ -179,10 +179,10 @@ static void tick_setup_device(struct tick_device *td,
> * this cpu:
> */
> if (tick_do_timer_cpu == TICK_DO_TIMER_BOOT) {
> - if (!tick_nohz_full_cpu(cpu))
> - tick_do_timer_cpu = cpu;
> + if (tick_nohz_full_enabled())
> + tick_do_timer_cpu = TICK_DO_TIMER_DEFAULT;
> else
> - tick_do_timer_cpu = TICK_DO_TIMER_NONE;
> + tick_do_timer_cpu = cpu;
> tick_next_period = ktime_get();
> tick_period = ktime_set(0, NSEC_PER_SEC / HZ);
> }
So from what I can tell this code can get called before SMP setup, which
would mean we could get here before CPU0 is online?
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