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Date: Mon, 26 Oct 2020 09:32:02 +0100 From: Vincent Guittot <vincent.guittot@...aro.org> To: Xi Wang <xii@...gle.com> Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, Juri Lelli <juri.lelli@...hat.com>, Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, Josh Don <joshdon@...gle.com>, linux-kernel <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v3 1/1] sched: watchdog: Touch kernel watchdog with sched count On Sun, 25 Oct 2020 at 00:57, Xi Wang <xii@...gle.com> wrote: > > 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. When a patch provides an improvement, it's usually good to give figures that show the improvement > > Suggested-by: Paul Turner <pjt@...gle.com> > Suggested-by: Peter Zijlstra <peterz@...radead.org> > Signed-off-by: Xi Wang <xii@...gle.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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