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Subject[PATCH v2 2/4] watchdog/softlockup: Replace "watchdog/%u" threads with cpu_stop_work
Oleg suggested to replace the "watchdog/%u" threads with
cpu_stop_work. That removes one thread per cpu while at the same time
fixes softlockup vs SCHED_DEADLINE.

But more importantly, it does away with the single
smpboot_update_cpumask_percpu_thread() user, which allows
cleanups/shrinkage of the smpboot interface.

Cc: Don Zickus <dzickus@redhat.com>
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/cpuhotplug.h | 1
include/linux/nmi.h | 5 +
kernel/cpu.c | 5 +
kernel/watchdog.c | 137 +++++++++++++++++++--------------------------
4 files changed, 71 insertions(+), 77 deletions(-)

--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -164,6 +164,7 @@ enum cpuhp_state {
CPUHP_AP_PERF_POWERPC_NEST_IMC_ONLINE,
CPUHP_AP_PERF_POWERPC_CORE_IMC_ONLINE,
CPUHP_AP_PERF_POWERPC_THREAD_IMC_ONLINE,
+ CPUHP_AP_WATCHDOG_ONLINE,
CPUHP_AP_WORKQUEUE_ONLINE,
CPUHP_AP_RCUTREE_ONLINE,
CPUHP_AP_ONLINE_DYN,
--- a/include/linux/nmi.h
+++ b/include/linux/nmi.h
@@ -33,10 +33,15 @@ extern int sysctl_hardlockup_all_cpu_bac
#define sysctl_hardlockup_all_cpu_backtrace 0
#endif /* !CONFIG_SMP */

+extern int lockup_detector_online_cpu(unsigned int cpu);
+extern int lockup_detector_offline_cpu(unsigned int cpu);
+
#else /* CONFIG_LOCKUP_DETECTOR */
static inline void lockup_detector_init(void) { }
static inline void lockup_detector_soft_poweroff(void) { }
static inline void lockup_detector_cleanup(void) { }
+#define lockup_detector_online_cpu NULL
+#define lockup_detector_offline_cpu NULL
#endif /* !CONFIG_LOCKUP_DETECTOR */

#ifdef CONFIG_SOFTLOCKUP_DETECTOR
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1344,6 +1344,11 @@ static struct cpuhp_step cpuhp_hp_states
.startup.single = perf_event_init_cpu,
.teardown.single = perf_event_exit_cpu,
},
+ [CPUHP_AP_WATCHDOG_ONLINE] = {
+ .name = "lockup_detector:online",
+ .startup.single = lockup_detector_online_cpu,
+ .teardown.single = lockup_detector_offline_cpu,
+ },
[CPUHP_AP_WORKQUEUE_ONLINE] = {
.name = "workqueue:online",
.startup.single = workqueue_online_cpu,
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -18,18 +18,14 @@
#include <linux/init.h>
#include <linux/module.h>
#include <linux/sysctl.h>
-#include <linux/smpboot.h>
-#include <linux/sched/rt.h>
-#include <uapi/linux/sched/types.h>
#include <linux/tick.h>
-#include <linux/workqueue.h>
#include <linux/sched/clock.h>
#include <linux/sched/debug.h>
#include <linux/sched/isolation.h>
+#include <linux/stop_machine.h>

#include <asm/irq_regs.h>
#include <linux/kvm_para.h>
-#include <linux/kthread.h>

static DEFINE_MUTEX(watchdog_mutex);

@@ -169,11 +165,10 @@ static void lockup_detector_update_enabl
unsigned int __read_mostly softlockup_panic =
CONFIG_BOOTPARAM_SOFTLOCKUP_PANIC_VALUE;

-static bool softlockup_threads_initialized __read_mostly;
+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(struct task_struct *, softlockup_watchdog);
static DEFINE_PER_CPU(struct hrtimer, watchdog_hrtimer);
static DEFINE_PER_CPU(bool, softlockup_touch_sync);
static DEFINE_PER_CPU(bool, soft_watchdog_warn);
@@ -335,6 +330,25 @@ static void watchdog_interrupt_count(voi
__this_cpu_inc(hrtimer_interrupts);
}

+/*
+ * 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)
+{
+ __this_cpu_write(soft_lockup_hrtimer_cnt,
+ __this_cpu_read(hrtimer_interrupts));
+ __touch_watchdog();
+
+ return 0;
+}
+
+static DEFINE_PER_CPU(struct cpu_stop_work, softlockup_stop_work);
+
/* watchdog kicker functions */
static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
{
@@ -350,7 +364,9 @@ static enum hrtimer_restart watchdog_tim
watchdog_interrupt_count();

/* kick the softlockup detector */
- wake_up_process(__this_cpu_read(softlockup_watchdog));
+ 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));
@@ -448,17 +464,12 @@ static enum hrtimer_restart watchdog_tim
return HRTIMER_RESTART;
}

-static void watchdog_set_prio(unsigned int policy, unsigned int prio)
-{
- struct sched_param param = { .sched_priority = prio };
-
- sched_setscheduler(current, policy, &param);
-}
-
static void watchdog_enable(unsigned int cpu)
{
struct hrtimer *hrtimer = this_cpu_ptr(&watchdog_hrtimer);

+ WARN_ON_ONCE(cpu != smp_processor_id());
+
/*
* Start the timer first to prevent the NMI watchdog triggering
* before the timer has a chance to fire.
@@ -473,15 +484,14 @@ static void watchdog_enable(unsigned int
/* Enable the perf event */
if (watchdog_enabled & NMI_WATCHDOG_ENABLED)
watchdog_nmi_enable(cpu);
-
- watchdog_set_prio(SCHED_FIFO, MAX_RT_PRIO - 1);
}

static void watchdog_disable(unsigned int cpu)
{
struct hrtimer *hrtimer = this_cpu_ptr(&watchdog_hrtimer);

- watchdog_set_prio(SCHED_NORMAL, 0);
+ WARN_ON_ONCE(cpu != smp_processor_id());
+
/*
* Disable the perf event first. That prevents that a large delay
* between disabling the timer and disabling the perf event causes
@@ -491,77 +501,63 @@ static void watchdog_disable(unsigned in
hrtimer_cancel(hrtimer);
}

-static void watchdog_cleanup(unsigned int cpu, bool online)
+static int softlockup_stop_fn(void *data)
{
- watchdog_disable(cpu);
+ watchdog_disable(smp_processor_id());
+ return 0;
}

-static int watchdog_should_run(unsigned int cpu)
+static void softlockup_stop_all(void)
{
- return __this_cpu_read(hrtimer_interrupts) !=
- __this_cpu_read(soft_lockup_hrtimer_cnt);
+ int cpu;
+
+ if (!softlockup_initialized)
+ return;
+
+ for_each_cpu(cpu, &watchdog_allowed_mask)
+ smp_call_on_cpu(cpu, softlockup_stop_fn, NULL, false);
+
+ cpumask_clear(&watchdog_allowed_mask);
}

-/*
- * 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 void watchdog(unsigned int cpu)
+static int softlockup_start_fn(void *data)
{
- __this_cpu_write(soft_lockup_hrtimer_cnt,
- __this_cpu_read(hrtimer_interrupts));
- __touch_watchdog();
+ watchdog_enable(smp_processor_id());
+ return 0;
}

-static struct smp_hotplug_thread watchdog_threads = {
- .store = &softlockup_watchdog,
- .thread_should_run = watchdog_should_run,
- .thread_fn = watchdog,
- .thread_comm = "watchdog/%u",
- .setup = watchdog_enable,
- .cleanup = watchdog_cleanup,
- .park = watchdog_disable,
- .unpark = watchdog_enable,
-};
-
-static void softlockup_update_smpboot_threads(void)
+static void softlockup_start_all(void)
{
- lockdep_assert_held(&watchdog_mutex);
-
- if (!softlockup_threads_initialized)
- return;
+ int cpu;

- smpboot_update_cpumask_percpu_thread(&watchdog_threads,
- &watchdog_allowed_mask);
+ cpumask_copy(&watchdog_allowed_mask, &watchdog_cpumask);
+ for_each_cpu(cpu, &watchdog_allowed_mask)
+ smp_call_on_cpu(cpu, softlockup_start_fn, NULL, false);
}

-/* Temporarily park all watchdog threads */
-static void softlockup_park_all_threads(void)
+int lockup_detector_online_cpu(unsigned int cpu)
{
- cpumask_clear(&watchdog_allowed_mask);
- softlockup_update_smpboot_threads();
+ watchdog_enable(cpu);
+ return 0;
}

-/* Unpark enabled threads */
-static void softlockup_unpark_threads(void)
+int lockup_detector_offline_cpu(unsigned int cpu)
{
- cpumask_copy(&watchdog_allowed_mask, &watchdog_cpumask);
- softlockup_update_smpboot_threads();
+ watchdog_disable(cpu);
+ return 0;
}

static void lockup_detector_reconfigure(void)
{
cpus_read_lock();
watchdog_nmi_stop();
- softlockup_park_all_threads();
+
+ softlockup_stop_all();
set_sample_period();
lockup_detector_update_enable();
if (watchdog_enabled && watchdog_thresh)
- softlockup_unpark_threads();
+ softlockup_start_all();
+
watchdog_nmi_start();
cpus_read_unlock();
/*
@@ -580,8 +576,6 @@ static void lockup_detector_reconfigure(
*/
static __init void lockup_detector_setup(void)
{
- int ret;
-
/*
* If sysctl is off and watchdog got disabled on the command line,
* nothing to do here.
@@ -592,24 +586,13 @@ static __init void lockup_detector_setup
!(watchdog_enabled && watchdog_thresh))
return;

- ret = smpboot_register_percpu_thread_cpumask(&watchdog_threads,
- &watchdog_allowed_mask);
- if (ret) {
- pr_err("Failed to initialize soft lockup detector threads\n");
- return;
- }
-
mutex_lock(&watchdog_mutex);
- softlockup_threads_initialized = true;
+ softlockup_initialized = true;
lockup_detector_reconfigure();
mutex_unlock(&watchdog_mutex);
}

#else /* CONFIG_SOFTLOCKUP_DETECTOR */
-static inline int watchdog_park_threads(void) { return 0; }
-static inline void watchdog_unpark_threads(void) { }
-static inline int watchdog_enable_all_cpus(void) { return 0; }
-static inline void watchdog_disable_all_cpus(void) { }
static void lockup_detector_reconfigure(void)
{
cpus_read_lock();

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