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From 413fbf5193b330c5f478ef7aaeaaee08907a993e Mon Sep 17 00:00:00 2001
From: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Date: Mon, 30 Mar 2015 14:59:19 +0530
Subject: [PATCH] clockevents: Fix cpu_down() race for hrtimer based broadcasting
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:
7cba160ad789 ("powernv/cpuidle: Redesign idle states management")
which exposed the cpu_down() race with hrtimer based broadcast mode:
5d1638acb9f6 ("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, tidied up the changelog. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
kernel/cpu.c | 2 ++
kernel/time/tick-broadcast.c | 19 +++++++++++--------
kernel/time/tick-internal.h | 2 ++
3 files changed, 15 insertions(+), 8 deletions(-)
@@ -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();
+ tick_takeover(cpu);
/* This actually kills the CPU. */
__cpu_die(cpu);
@@ -680,14 +680,19 @@ 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 tick_takeover(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 */
- clockevents_program_event(bc, bc->next_event, 1);
+ 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);
}
/*
@@ -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);
}
@@ -126,6 +126,7 @@ extern int tick_broadcast_oneshot_active(void);
extern void tick_check_oneshot_broadcast_this_cpu(void);
bool tick_broadcast_oneshot_available(void);
extern struct cpumask *tick_get_broadcast_oneshot_mask(void);
+extern void tick_takeover(int deadcpu);
#else /* !(BROADCAST && ONESHOT): */
static inline void tick_broadcast_setup_oneshot(struct clock_event_device *bc) { BUG(); }
static inline int tick_broadcast_oneshot_control(unsigned long reason) { return 0; }
@@ -134,6 +135,7 @@ static inline void tick_shutdown_broadcast_oneshot(unsigned int *cpup) { }
static inline int tick_broadcast_oneshot_active(void) { return 0; }
static inline void tick_check_oneshot_broadcast_this_cpu(void) { }
static inline bool tick_broadcast_oneshot_available(void) { return tick_oneshot_possible(); }
+static inline void tick_takeover(int deadcpu) { }
#endif /* !(BROADCAST && ONESHOT) */
/* NO_HZ_FULL internal */