On Monday, February 16, 2015 01:14:36 PM Peter Zijlstra wrote:
> From: Thomas Gleixner <t...@linutronix.de>
>
> While looking through the (ab)use of the clockevents_notify() function
> I stumbled over the following gem in the acpi_pad code:
>
> if (lapic_detected_unstable && !lapic_marked_unstable) {
> /* LAPIC could halt in idle, so notify users */
> for_each_online_cpu(i)
> clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_ON, &i);
> lapic_marked_unstable = 1;
> }
>
> This code calls on the cpu which detects the lapic unstable condition
> first clockevents_notify() to tell the core code that the broadcast
> should be enabled on all online cpus. Brilliant stuff that as it
> notifies the core code a num_online_cpus() times that the broadcast
> should be enabled on the current cpu.
>
> This probably has never been noticed because that code got never
> tested with NOHZ=n and HIGHRES_TIMER=n or it just worked by chance
> because one of the other mechanisms told the core in the right way
> that the local apic timer is wreckaged.
>
> Sigh, this is:
>
> - The 4th incarnation of idle drivers which has their own mechanism
> to detect and deal with X86_FEATURE_ARAT.
>
> - The 2nd incarnation of fake idle mechanisms with a different set of
> brainmelting bugs.
>
> - Has been merged against an explicit NAK of the scheduler
> maintainer with the promise to improve it over time.
>
> - Another example of featuritis driven trainwreck engineering.
>
> - Another pointless waste of my time.
>
> Fix this nonsense by removing that lapic detection and notification
> logic and simply call into the clockevents code unconditonally. The
> ARAT feature is marked in the lapic clockevent already so the core
> code will just ignore the requests and return.
>
> Signed-off-by: Thomas Gleixner <t...@linutronix.de>
> Cc: "Rafael J. Wysocki" <r...@rjwysocki.net>
Acked-by: Rafael J. Wysocki <rafael.j.wyso...@intel.com>
> Cc: Len Brown <l...@kernel.org>
> Cc: linux-a...@vger.kernel.org
> Cc: Peter Zijlstra <pet...@infradead.org>
> ---
> drivers/acpi/acpi_pad.c | 26 +++++---------------------
> 1 file changed, 5 insertions(+), 21 deletions(-)
>
> Index: linux/drivers/acpi/acpi_pad.c
> ===================================================================
> --- linux.orig/drivers/acpi/acpi_pad.c
> +++ linux/drivers/acpi/acpi_pad.c
> @@ -41,8 +41,6 @@ static unsigned long power_saving_mwait_
>
> static unsigned char tsc_detected_unstable;
> static unsigned char tsc_marked_unstable;
> -static unsigned char lapic_detected_unstable;
> -static unsigned char lapic_marked_unstable;
>
> static void power_saving_mwait_init(void)
> {
> @@ -82,13 +80,10 @@ static void power_saving_mwait_init(void
> */
> if (!boot_cpu_has(X86_FEATURE_NONSTOP_TSC))
> tsc_detected_unstable = 1;
> - if (!boot_cpu_has(X86_FEATURE_ARAT))
> - lapic_detected_unstable = 1;
> break;
> default:
> - /* TSC & LAPIC could halt in idle */
> + /* TSC could halt in idle */
> tsc_detected_unstable = 1;
> - lapic_detected_unstable = 1;
> }
> #endif
> }
> @@ -177,28 +172,17 @@ static int power_saving_thread(void *dat
> mark_tsc_unstable("TSC halts in idle");
> tsc_marked_unstable = 1;
> }
> - if (lapic_detected_unstable && !lapic_marked_unstable) {
> - int i;
> - /* LAPIC could halt in idle, so notify users */
> - for_each_online_cpu(i)
> - clockevents_notify(
> - CLOCK_EVT_NOTIFY_BROADCAST_ON,
> - &i);
> - lapic_marked_unstable = 1;
> - }
> + clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_ON, &cpu);
> +
> local_irq_disable();
> cpu = smp_processor_id();
> - if (lapic_marked_unstable)
> - clockevents_notify(
> - CLOCK_EVT_NOTIFY_BROADCAST_ENTER, &cpu);
> + clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_ENTER,
> &cpu);
> stop_critical_timings();
>
> mwait_idle_with_hints(power_saving_mwait_eax, 1);
>
> start_critical_timings();
> - if (lapic_marked_unstable)
> - clockevents_notify(
> - CLOCK_EVT_NOTIFY_BROADCAST_EXIT, &cpu);
> + clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_EXIT,
> &cpu);
> local_irq_enable();
>
> if (time_before(expire_time, jiffies)) {
>
>
> --
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--
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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