[PATCH v3 1/1] sched: watchdog: Touch kernel watchdog with sched count
From: Xi Wang
Date: Sat Oct 24 2020 - 20:59:50 EST
Instead of periodically resetting watchdogs from thread context,
this patch simply forces resched and checks rq->sched_count.
Watchdog is reset if the sched count increases. If the same thread
is picked by pick_next_task during resched, there is no context
switch.
With the new method we lose coverage on: a migration/n thread
actually gets picked and we actually context switch to the
migration/n thread. These steps are unlikely to silently fail.
The change would provide nearly the same level of protection with
less latency / jitter.
Suggested-by: Paul Turner <pjt@xxxxxxxxxx>
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Xi Wang <xii@xxxxxxxxxx>
---
include/linux/sched.h | 4 ++++
kernel/sched/core.c | 23 +++++++++++++++++++--
kernel/sched/sched.h | 6 +++++-
kernel/watchdog.c | 47 +++++++++++++------------------------------
4 files changed, 44 insertions(+), 36 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index d383cf09e78f..1e3bef9a9cdb 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1662,6 +1662,10 @@ extern int sched_setattr(struct task_struct *, const struct sched_attr *);
extern int sched_setattr_nocheck(struct task_struct *, const struct sched_attr *);
extern struct task_struct *idle_task(int cpu);
+#ifdef CONFIG_SOFTLOCKUP_DETECTOR
+extern unsigned int sched_get_count(int cpu);
+#endif
+
/**
* is_idle_task - is the specified task an idle task?
* @p: the task in question.
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 8160ab5263f8..378f0f36c402 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4293,8 +4293,6 @@ static inline void schedule_debug(struct task_struct *prev, bool preempt)
rcu_sleep_check();
profile_hit(SCHED_PROFILING, __builtin_return_address(0));
-
- schedstat_inc(this_rq()->sched_count);
}
static void put_prev_task_balance(struct rq *rq, struct task_struct *prev,
@@ -4492,6 +4490,12 @@ static void __sched notrace __schedule(bool preempt)
clear_tsk_need_resched(prev);
clear_preempt_need_resched();
+#ifdef CONFIG_SOFTLOCKUP_DETECTOR
+ this_rq()->sched_count++; /* sched count is also used by watchdog */
+#else
+ schedstat_inc(this_rq()->sched_count);
+#endif
+
if (likely(prev != next)) {
rq->nr_switches++;
/*
@@ -5117,6 +5121,21 @@ struct task_struct *idle_task(int cpu)
return cpu_rq(cpu)->idle;
}
+#ifdef CONFIG_SOFTLOCKUP_DETECTOR
+
+/**
+ * sched_get_count - get the sched count of a CPU.
+ * @cpu: the CPU in question.
+ *
+ * Return: sched count.
+ */
+unsigned int sched_get_count(int cpu)
+{
+ return cpu_rq(cpu)->sched_count;
+}
+
+#endif
+
/**
* find_process_by_pid - find a process with a matching PID value.
* @pid: the pid in question.
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 28709f6b0975..f23255981d52 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -959,6 +959,11 @@ struct rq {
u64 clock_pelt;
unsigned long lost_idle_time;
+#if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_SOFTLOCKUP_DETECTOR)
+ /* Also used by watchdog - no longer grouping with other sched stats */
+ unsigned int sched_count;
+#endif
+
atomic_t nr_iowait;
#ifdef CONFIG_MEMBARRIER
@@ -1036,7 +1041,6 @@ struct rq {
unsigned int yld_count;
/* schedule() stats */
- unsigned int sched_count;
unsigned int sched_goidle;
/* try_to_wake_up() stats */
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 5abb5b22ad13..22f87aded95a 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -170,6 +170,7 @@ static bool softlockup_initialized __read_mostly;
static u64 __read_mostly sample_period;
static DEFINE_PER_CPU(unsigned long, watchdog_touch_ts);
+static DEFINE_PER_CPU(unsigned int, watchdog_sched_prev);
static DEFINE_PER_CPU(struct hrtimer, watchdog_hrtimer);
static DEFINE_PER_CPU(bool, softlockup_touch_sync);
static DEFINE_PER_CPU(bool, soft_watchdog_warn);
@@ -239,6 +240,7 @@ static void set_sample_period(void)
static void __touch_watchdog(void)
{
__this_cpu_write(watchdog_touch_ts, get_timestamp());
+ __this_cpu_write(watchdog_sched_prev, sched_get_count(smp_processor_id()));
}
/**
@@ -318,25 +320,6 @@ static void watchdog_interrupt_count(void)
__this_cpu_inc(hrtimer_interrupts);
}
-static DEFINE_PER_CPU(struct completion, softlockup_completion);
-static DEFINE_PER_CPU(struct cpu_stop_work, softlockup_stop_work);
-
-/*
- * The watchdog thread function - touches the timestamp.
- *
- * It only runs once every sample_period seconds (4 seconds by
- * default) to reset the softlockup timestamp. If this gets delayed
- * for more than 2*watchdog_thresh seconds then the debug-printout
- * triggers in watchdog_timer_fn().
- */
-static int softlockup_fn(void *data)
-{
- __touch_watchdog();
- complete(this_cpu_ptr(&softlockup_completion));
-
- return 0;
-}
-
/* watchdog kicker functions */
static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
{
@@ -351,15 +334,6 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
/* kick the hardlockup detector */
watchdog_interrupt_count();
- /* kick the softlockup detector */
- if (completion_done(this_cpu_ptr(&softlockup_completion))) {
- reinit_completion(this_cpu_ptr(&softlockup_completion));
- stop_one_cpu_nowait(smp_processor_id(),
- softlockup_fn, NULL,
- this_cpu_ptr(&softlockup_stop_work));
- }
-
- /* .. and repeat */
hrtimer_forward_now(hrtimer, ns_to_ktime(sample_period));
if (touch_ts == SOFTLOCKUP_RESET) {
@@ -378,6 +352,18 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
return HRTIMER_RESTART;
}
+ /* Trigger reschedule for the next round */
+ set_tsk_need_resched(current);
+ set_preempt_need_resched();
+
+ /* sched_count increase in __schedule is taken as watchdog touched */
+ if (sched_get_count(smp_processor_id()) -
+ __this_cpu_read(watchdog_sched_prev)) {
+ __touch_watchdog();
+ __this_cpu_write(soft_watchdog_warn, false);
+ return HRTIMER_RESTART;
+ }
+
/* check for a softlockup
* This is done by making sure a high priority task is
* being scheduled. The task touches the watchdog to
@@ -443,13 +429,9 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
static void watchdog_enable(unsigned int cpu)
{
struct hrtimer *hrtimer = this_cpu_ptr(&watchdog_hrtimer);
- struct completion *done = this_cpu_ptr(&softlockup_completion);
WARN_ON_ONCE(cpu != smp_processor_id());
- init_completion(done);
- complete(done);
-
/*
* Start the timer first to prevent the NMI watchdog triggering
* before the timer has a chance to fire.
@@ -479,7 +461,6 @@ static void watchdog_disable(unsigned int cpu)
*/
watchdog_nmi_disable(cpu);
hrtimer_cancel(hrtimer);
- wait_for_completion(this_cpu_ptr(&softlockup_completion));
}
static int softlockup_stop_fn(void *data)
--
2.29.0.rc2.309.g374f81d7ae-goog