clockevents: Fix cpu_down() race for hrtimer based broadcasting (345527b1) · Commits · 戴 / test

It was found when doing a hotplug stress test on POWER, that the machine either hit softlockups or rcu_sched stall warnings. The issue was traced to commit: ("powernv/cpuidle: Redesign idle states management") which exposed the cpu_down() race with hrtimer based broadcast mode: ("tick: Introduce hrtimer based broadcast") The race is the following: Assume CPU1 is the CPU which holds the hrtimer broadcasting duty before it is taken down. CPU0 CPU1 cpu_down() take_cpu_down() disable_interrupts() cpu_die() while (CPU1 != CPU_DEAD) { msleep(100); switch_to_idle(); stop_cpu_timer(); schedule_broadcast(); } tick_cleanup_cpu_dead() take_over_broadcast() So after CPU1 disabled interrupts it cannot handle the broadcast hrtimer anymore, so CPU0 will be stuck forever. Fix this by explicitly taking over broadcast duty before cpu_die(). This is a temporary workaround. What we really want is a callback in the clockevent device which allows us to do that from the dying CPU by pushing the hrtimer onto a different cpu. That might involve an IPI and is definitely more complex than this immediate fix. Changelog was picked up from: https://lkml.org/lkml/2015/2/16/213 Suggested-by:

Thomas Gleixner <tglx@linutronix.de> Tested-by:

Nicolas Pitre <nico@linaro.org> Signed-off-by:

Preeti U. Murthy <preeti@linux.vnet.ibm.com> Cc: linuxppc-dev@lists.ozlabs.org Cc: mpe@ellerman.id.au Cc: nicolas.pitre@linaro.org Cc: peterz@infradead.org Cc: rjw@rjwysocki.net Fixes: http://linuxppc.10917.n7.nabble.com/offlining-cpus-breakage-td88619.html Link: http://lkml.kernel.org/r/20150330092410.24979.59887.stgit@preeti.in.ibm.com [ Merged it to the latest timer tree, renamed the callback, tidied up the changelog. ] Signed-off-by:

Ingo Molnar <mingo@kernel.org>

include/linux/tick.h

+6 −0

Original line number	Diff line number	Diff line
		@@ -36,6 +36,12 @@ extern void tick_irq_enter(void);
		static inline void tick_irq_enter(void) { }
		#endif

		#if defined(CONFIG_GENERIC_CLOCKEVENTS_BROADCAST) && defined(CONFIG_TICK_ONESHOT)
		extern void hotplug_cpu__broadcast_tick_pull(int dead_cpu);
		#else
		static inline void hotplug_cpu__broadcast_tick_pull(int dead_cpu) { }
		#endif

		#ifdef CONFIG_NO_HZ_COMMON
		extern int tick_nohz_tick_stopped(void);
		extern void tick_nohz_idle_enter(void);

kernel/cpu.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -20,6 +20,7 @@
		#include <linux/gfp.h>
		#include <linux/suspend.h>
		#include <linux/lockdep.h>
		#include <linux/tick.h>
		#include <trace/events/power.h>

		#include "smpboot.h"
		@@ -411,6 +412,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
		while (!idle_cpu(cpu))
		cpu_relax();

		hotplug_cpu__broadcast_tick_pull(cpu);
		/* This actually kills the CPU. */
		__cpu_die(cpu);

kernel/time/tick-broadcast.c

+11 −8

Original line number	Diff line number	Diff line
		@@ -680,15 +680,20 @@ static void broadcast_shutdown_local(struct clock_event_device *bc,
		clockevents_set_state(dev, CLOCK_EVT_STATE_SHUTDOWN);
		}

		static void broadcast_move_bc(int deadcpu)
		void hotplug_cpu__broadcast_tick_pull(int deadcpu)
		{
		struct clock_event_device *bc = tick_broadcast_device.evtdev;
		struct clock_event_device *bc;
		unsigned long flags;

		if (!bc \|\| !broadcast_needs_cpu(bc, deadcpu))
		return;
		/* This moves the broadcast assignment to this cpu */
		raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
		bc = tick_broadcast_device.evtdev;

		if (bc && broadcast_needs_cpu(bc, deadcpu)) {
		/* This moves the broadcast assignment to this CPU: */
		clockevents_program_event(bc, bc->next_event, 1);
		}
		raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags);
		}

		/*
		* Powerstate information: The system enters/leaves a state, where
		@@ -924,8 +929,6 @@ void tick_shutdown_broadcast_oneshot(unsigned int *cpup)
		cpumask_clear_cpu(cpu, tick_broadcast_pending_mask);
		cpumask_clear_cpu(cpu, tick_broadcast_force_mask);

		broadcast_move_bc(cpu);

		raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags);
		}

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