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Subject[patch 2/4] percpu enable flag for softlockup watchdog
On a NO_HZ system, there may be an arbitrarily long delay between
ticks on a CPU. When we're disabling ticks for a CPU, also disable
the softlockup watchdog timer.

This makes the touch_softlockup_watchdog() interface redundant; if a
piece of code knows its going to be holding off timer interrupts long
enough to trigger the watchdog, then it may as well simply temporarily
disable the timer.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Zachary Amsden <zach@vmware.com>
Cc: James Morris <jmorris@namei.org>
Cc: Dan Hecht <dhecht@vmware.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Chris Lalancette <clalance@redhat.com>
Cc: Eric Dumazet <dada1@cosmosbay.com>

---
include/linux/sched.h | 25 +++++++++++++++++
kernel/softlockup.c | 67 ++++++++++++++++++++++++++++++++++++++++++----
kernel/time/tick-sched.c | 34 ++++++++++-------------
3 files changed, 102 insertions(+), 24 deletions(-)
===================================================================
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -232,12 +232,37 @@ extern void scheduler_tick(void);
extern void scheduler_tick(void);

#ifdef CONFIG_DETECT_SOFTLOCKUP
+/* A scheduler tick on this CPU */
extern void softlockup_tick(void);
+
+/* Scheduler is disabling/re-enabling ticks on this CPU */
+extern void softlockup_tick_disable(void);
+extern void softlockup_tick_enable(void);
+
+/* Some code wants to temporarily disable the watchdog */
+extern int softlockup_disable(void);
+extern void softlockup_enable(int state);
+
extern void spawn_softlockup_task(void);
extern void touch_softlockup_watchdog(void);
#else
static inline void softlockup_tick(void)
{
+}
+static inline void softlockup_tick_disable(void)
+{
+}
+static inline void softlockup_tick_enable(void)
+{
+}
+static inline int softlockup_disable(void)
+{
+ preempt_disable();
+ return 0;
+}
+static inline void softlockup_enable(int state)
+{
+ preempt_enable();
}
static inline void spawn_softlockup_task(void)
{
===================================================================
--- a/kernel/softlockup.c
+++ b/kernel/softlockup.c
@@ -20,6 +20,7 @@ static DEFINE_PER_CPU(unsigned long, tou
static DEFINE_PER_CPU(unsigned long, touch_timestamp);
static DEFINE_PER_CPU(unsigned long, print_timestamp);
static DEFINE_PER_CPU(struct task_struct *, watchdog_task);
+static DEFINE_PER_CPU(int, enabled);

static int did_panic = 0;

@@ -45,11 +46,65 @@ static unsigned long get_timestamp(void)
return sched_clock() >> 30; /* 2^30 ~= 10^9 */
}

-void touch_softlockup_watchdog(void)
+void inline touch_softlockup_watchdog(void)
{
__raw_get_cpu_var(touch_timestamp) = get_timestamp();
}
EXPORT_SYMBOL(touch_softlockup_watchdog);
+
+/*
+ * Disable the watchdog on this CPU. This is called directly by the
+ * scheduler to tell us it's going tickless.
+ */
+void inline softlockup_tick_disable(void)
+{
+ __get_cpu_var(enabled) = 0;
+}
+
+/*
+ * Disable the watchdog for this CPU, returning the current state to
+ * allow nesting. Returns with preemptio disabled, since we can't
+ * switch CPUs before we re-enable the watchdog (also, if we're
+ * worried about getting watchdog timeouts, we're not scheduling).
+ */
+int softlockup_disable(void)
+{
+ int ret;
+
+ preempt_disable();
+
+ ret = __get_cpu_var(enabled);
+ softlockup_tick_disable();
+
+ return ret;
+}
+EXPORT_SYMBOL(softlockup_disable);
+
+/*
+ * Re-enable the watchdog on this CPU. Called directly by the
+ * scheduler to tell us ticks are resuming.
+ */
+void inline softlockup_tick_enable(void)
+{
+ __get_cpu_var(enabled) = 1;
+}
+
+/*
+ * Returns softlockup watchdog state to before the last per-cpu
+ * disable.
+ */
+void softlockup_enable(int state)
+{
+ if (state) {
+ touch_softlockup_watchdog();
+ /* update timestamp before enable */
+ barrier();
+ softlockup_tick_enable();
+ }
+
+ preempt_enable();
+}
+EXPORT_SYMBOL(softlockup_enable);

/*
* This callback runs from the timer interrupt, and checks
@@ -62,8 +117,8 @@ void softlockup_tick(void)
unsigned long print_timestamp;
unsigned long now;

- /* watchdog task hasn't updated timestamp yet */
- if (touch_timestamp == 0)
+ /* return if not enabled */
+ if (!__get_cpu_var(enabled))
return;

print_timestamp = __get_cpu_var(print_timestamp);
@@ -108,8 +163,8 @@ static int watchdog(void * __bind_cpu)
sched_setscheduler(current, SCHED_FIFO, &param);
current->flags |= PF_NOFREEZE;

- /* initialize timestamp */
- touch_softlockup_watchdog();
+ /* enable on this cpu */
+ softlockup_tick_enable();

/*
* Run briefly once per second to reset the softlockup timestamp.
@@ -122,6 +177,8 @@ static int watchdog(void * __bind_cpu)
touch_softlockup_watchdog();
schedule();
}
+
+ softlockup_tick_disable();

return 0;
}
===================================================================
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -228,6 +228,8 @@ void tick_nohz_stop_sched_tick(void)
ts->idle_tick = ts->sched_timer.expires;
ts->tick_stopped = 1;
ts->idle_jiffies = last_jiffies;
+
+ softlockup_tick_disable();
}
/*
* calculate the expiry time for the next timer wheel
@@ -255,6 +257,7 @@ void tick_nohz_stop_sched_tick(void)
cpu_clear(cpu, nohz_cpu_mask);
}
raise_softirq_irqoff(TIMER_SOFTIRQ);
+
out:
ts->next_jiffies = next_jiffies;
ts->last_jiffies = last_jiffies;
@@ -311,6 +314,8 @@ void tick_nohz_restart_sched_tick(void)
ts->tick_stopped = 0;
hrtimer_cancel(&ts->sched_timer);
ts->sched_timer.expires = ts->idle_tick;
+
+ softlockup_tick_enable();

while (1) {
/* Forward the time to expire in the future */
@@ -355,17 +360,12 @@ static void tick_nohz_handler(struct clo
tick_do_update_jiffies64(now);

/*
- * When we are idle and the tick is stopped, we have to touch
- * the watchdog as we might not schedule for a really long
- * time. This happens on complete idle SMP systems while
- * waiting on the login prompt. We also increment the "start
- * of idle" jiffy stamp so the idle accounting adjustment we
- * do when we go busy again does not account too much ticks.
- */
- if (ts->tick_stopped) {
- touch_softlockup_watchdog();
+ * Increment the "start of idle" jiffy stamp so the idle
+ * accounting adjustment we do when we go busy again does not
+ * account too much ticks.
+ */
+ if (ts->tick_stopped)
ts->idle_jiffies++;
- }

update_process_times(user_mode(regs));
profile_tick(CPU_PROFILING);
@@ -450,17 +450,12 @@ static enum hrtimer_restart tick_sched_t
*/
if (regs) {
/*
- * When we are idle and the tick is stopped, we have to touch
- * the watchdog as we might not schedule for a really long
- * time. This happens on complete idle SMP systems while
- * waiting on the login prompt. We also increment the "start of
- * idle" jiffy stamp so the idle accounting adjustment we do
- * when we go busy again does not account too much ticks.
+ * Increment the "start of idle" jiffy stamp so the
+ * idle accounting adjustment we do when we go busy
+ * again does not account too much ticks.
*/
- if (ts->tick_stopped) {
- touch_softlockup_watchdog();
+ if (ts->tick_stopped)
ts->idle_jiffies++;
- }
/*
* update_process_times() might take tasklist_lock, hence
* drop the base lock. sched-tick hrtimers are per-CPU and
@@ -522,6 +517,7 @@ void tick_cancel_sched_timer(int cpu)
if (ts->sched_timer.base)
hrtimer_cancel(&ts->sched_timer);
ts->tick_stopped = 0;
+ softlockup_tick_enable();
ts->nohz_mode = NOHZ_MODE_INACTIVE;
}
#endif /* HIGH_RES_TIMERS */
--

-
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