Messages in this thread Patch in this message | | | Date | Sat, 24 Oct 2020 15:57:16 -0700 | Subject | [PATCH v3 1/1] sched: watchdog: Touch kernel watchdog with sched count | From | Xi Wang <> |
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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@google.com> Suggested-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Xi Wang <xii@google.com> --- 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
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