| Message ID | 20250117232433.24027-1-frederic@kernel.org (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | ff3be3e9ffdbcefcb0355ec1cd05342e92463ecb |
| Headers | show |
| Series | [v4] hrtimers: Force migrate away hrtimers queued after CPUHP_AP_HRTIMERS_DYING | expand |
On Sat, Jan 18, 2025 at 12:24:33AM +0100, Frederic Weisbecker wrote: > hrtimers are migrated away from the dying CPU to any online target at > the CPUHP_AP_HRTIMERS_DYING stage in order not to delay bandwidth timers > handling tasks involved in the CPU hotplug forward progress. > > However wake ups can still be performed by the outgoing CPU after > CPUHP_AP_HRTIMERS_DYING. Those can result again in bandwidth timers > being armed. Depending on several considerations (crystal ball > power management based election, earliest timer already enqueued, timer > migration enabled or not), the target may eventually be the current > CPU even if offline. If that happens, the timer is eventually ignored. > > The most notable example is RCU which had to deal with each and every of > those wake-ups by deferring them to an online CPU, along with related > workarounds: > > _ e787644caf76 (rcu: Defer RCU kthreads wakeup when CPU is dying) > _ 9139f93209d1 (rcu/nocb: Fix RT throttling hrtimer armed from offline CPU) > _ f7345ccc62a4 (rcu/nocb: Fix rcuog wake-up from offline softirq) > > The problem isn't confined to RCU though as the stop machine kthread > (which runs CPUHP_AP_HRTIMERS_DYING) reports its completion at the end > of its work through cpu_stop_signal_done() and performs a wake up that > eventually arms the deadline server timer: > > WARNING: CPU: 94 PID: 588 at kernel/time/hrtimer.c:1086 hrtimer_start_range_ns+0x289/0x2d0 > CPU: 94 UID: 0 PID: 588 Comm: migration/94 Not tainted > Stopper: multi_cpu_stop+0x0/0x120 <- stop_machine_cpuslocked+0x66/0xc0 > RIP: 0010:hrtimer_start_range_ns+0x289/0x2d0 > Call Trace: > <TASK> > ? hrtimer_start_range_ns > start_dl_timer > enqueue_dl_entity > dl_server_start > enqueue_task_fair > enqueue_task > ttwu_do_activate > try_to_wake_up > complete > cpu_stopper_thread > smpboot_thread_fn > kthread > ret_from_fork > ret_from_fork_asm > </TASK> > > Instead of providing yet another bandaid to work around the situation, > fix it from hrtimers infrastructure instead: always migrate away a > timer to an online target whenever it is enqueued from an offline CPU. > > This will also allow to revert all the above RCU disgraceful hacks. > > Reported-by: Vlad Poenaru <vlad.wing@gmail.com> > Reported-by: Usama Arif <usamaarif642@gmail.com> > Fixes: 5c0930ccaad5 ("hrtimers: Push pending hrtimers away from outgoing CPU earlier") > Closes: 20241213203739.1519801-1-usamaarif642@gmail.com > Signed-off-by: Frederic Weisbecker <frederic@kernel.org> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org> This passes over-holiday testing rcutorture, so, perhaps redundantly: Tested-by: Paul E. McKenney <paulmck@kernel.org> > --- > include/linux/hrtimer_defs.h | 1 + > kernel/time/hrtimer.c | 92 +++++++++++++++++++++++++++++------- > 2 files changed, 75 insertions(+), 18 deletions(-) > > diff --git a/include/linux/hrtimer_defs.h b/include/linux/hrtimer_defs.h > index c3b4b7ed7c16..84a5045f80f3 100644 > --- a/include/linux/hrtimer_defs.h > +++ b/include/linux/hrtimer_defs.h > @@ -125,6 +125,7 @@ struct hrtimer_cpu_base { > ktime_t softirq_expires_next; > struct hrtimer *softirq_next_timer; > struct hrtimer_clock_base clock_base[HRTIMER_MAX_CLOCK_BASES]; > + call_single_data_t csd; > } ____cacheline_aligned; > > > diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c > index 030426c8c944..082d4b687fa1 100644 > --- a/kernel/time/hrtimer.c > +++ b/kernel/time/hrtimer.c > @@ -58,6 +58,8 @@ > #define HRTIMER_ACTIVE_SOFT (HRTIMER_ACTIVE_HARD << MASK_SHIFT) > #define HRTIMER_ACTIVE_ALL (HRTIMER_ACTIVE_SOFT | HRTIMER_ACTIVE_HARD) > > +static void retrigger_next_event(void *arg); > + > /* > * The timer bases: > * > @@ -111,7 +113,8 @@ DEFINE_PER_CPU(struct hrtimer_cpu_base, hrtimer_bases) = > .clockid = CLOCK_TAI, > .get_time = &ktime_get_clocktai, > }, > - } > + }, > + .csd = CSD_INIT(retrigger_next_event, NULL) > }; > > static const int hrtimer_clock_to_base_table[MAX_CLOCKS] = { > @@ -124,6 +127,14 @@ static const int hrtimer_clock_to_base_table[MAX_CLOCKS] = { > [CLOCK_TAI] = HRTIMER_BASE_TAI, > }; > > +static inline bool hrtimer_base_is_online(struct hrtimer_cpu_base *base) > +{ > + if (!IS_ENABLED(CONFIG_HOTPLUG_CPU)) > + return true; > + else > + return likely(base->online); > +} > + > /* > * Functions and macros which are different for UP/SMP systems are kept in a > * single place > @@ -183,27 +194,58 @@ struct hrtimer_clock_base *lock_hrtimer_base(const struct hrtimer *timer, > } > > /* > - * We do not migrate the timer when it is expiring before the next > - * event on the target cpu. When high resolution is enabled, we cannot > - * reprogram the target cpu hardware and we would cause it to fire > - * late. To keep it simple, we handle the high resolution enabled and > - * disabled case similar. > + * Check if the elected target is suitable considering its next > + * event and the hotplug state of the current CPU. > + * > + * If the elected target is remote and its next event is after the timer > + * to queue, then a remote reprogram is necessary. However there is no > + * guarantee the IPI handling the operation would arrive in time to meet > + * the high resolution deadline. In this case the local CPU becomes a > + * preferred target, unless it is offline. > + * > + * High and low resolution modes are handled the same way for simplicity. > * > * Called with cpu_base->lock of target cpu held. > */ > -static int > -hrtimer_check_target(struct hrtimer *timer, struct hrtimer_clock_base *new_base) > +static bool > +hrtimer_suitable_target(struct hrtimer *timer, > + struct hrtimer_clock_base *new_base, > + struct hrtimer_cpu_base *new_cpu_base, > + struct hrtimer_cpu_base *this_cpu_base) > { > ktime_t expires; > > + /* > + * The local CPU clockevent can be reprogrammed. Also get_target_base() > + * guarantees it is online. > + */ > + if (new_cpu_base == this_cpu_base) > + return true; > + > + /* > + * The offline local CPU can't be the default target if the > + * next remote target event is after this timer. Keep the > + * elected new base. An IPI will we issued to reprogram > + * it as a last resort. > + */ > + if (!hrtimer_base_is_online(this_cpu_base)) > + return true; > + > expires = ktime_sub(hrtimer_get_expires(timer), new_base->offset); > - return expires < new_base->cpu_base->expires_next; > + > + return expires >= new_base->cpu_base->expires_next; > } > > static inline > struct hrtimer_cpu_base *get_target_base(struct hrtimer_cpu_base *base, > int pinned) > { > + if (!hrtimer_base_is_online(base)) { > + int cpu = cpumask_any_and(cpu_online_mask, housekeeping_cpumask(HK_TYPE_TIMER)); > + > + return &per_cpu(hrtimer_bases, cpu); > + } > + > #if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON) > if (static_branch_likely(&timers_migration_enabled) && !pinned) > return &per_cpu(hrtimer_bases, get_nohz_timer_target()); > @@ -254,8 +296,8 @@ switch_hrtimer_base(struct hrtimer *timer, struct hrtimer_clock_base *base, > raw_spin_unlock(&base->cpu_base->lock); > raw_spin_lock(&new_base->cpu_base->lock); > > - if (new_cpu_base != this_cpu_base && > - hrtimer_check_target(timer, new_base)) { > + if (!hrtimer_suitable_target(timer, new_base, new_cpu_base, > + this_cpu_base)) { > raw_spin_unlock(&new_base->cpu_base->lock); > raw_spin_lock(&base->cpu_base->lock); > new_cpu_base = this_cpu_base; > @@ -264,8 +306,8 @@ switch_hrtimer_base(struct hrtimer *timer, struct hrtimer_clock_base *base, > } > WRITE_ONCE(timer->base, new_base); > } else { > - if (new_cpu_base != this_cpu_base && > - hrtimer_check_target(timer, new_base)) { > + if (!hrtimer_suitable_target(timer, new_base, new_cpu_base, > + this_cpu_base)) { > new_cpu_base = this_cpu_base; > goto again; > } > @@ -716,8 +758,6 @@ static inline int hrtimer_is_hres_enabled(void) > return hrtimer_hres_enabled; > } > > -static void retrigger_next_event(void *arg); > - > /* > * Switch to high resolution mode > */ > @@ -1206,6 +1246,7 @@ static int __hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim, > u64 delta_ns, const enum hrtimer_mode mode, > struct hrtimer_clock_base *base) > { > + struct hrtimer_cpu_base *this_cpu_base = this_cpu_ptr(&hrtimer_bases); > struct hrtimer_clock_base *new_base; > bool force_local, first; > > @@ -1217,9 +1258,15 @@ static int __hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim, > * and enforce reprogramming after it is queued no matter whether > * it is the new first expiring timer again or not. > */ > - force_local = base->cpu_base == this_cpu_ptr(&hrtimer_bases); > + force_local = base->cpu_base == this_cpu_base; > force_local &= base->cpu_base->next_timer == timer; > > + /* > + * Don't force local queuing if this enqueue happens on a unplugged > + * CPU after hrtimer_cpu_dying() has been invoked. > + */ > + force_local &= this_cpu_base->online; > + > /* > * Remove an active timer from the queue. In case it is not queued > * on the current CPU, make sure that remove_hrtimer() updates the > @@ -1249,8 +1296,17 @@ static int __hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim, > } > > first = enqueue_hrtimer(timer, new_base, mode); > - if (!force_local) > - return first; > + if (!force_local) { > + if (hrtimer_base_is_online(this_cpu_base)) > + return first; > + > + if (first) { > + struct hrtimer_cpu_base *new_cpu_base = new_base->cpu_base; > + > + smp_call_function_single_async(new_cpu_base->cpu, &new_cpu_base->csd); > + } > + return 0; > + } > > /* > * Timer was forced to stay on the current CPU to avoid > -- > 2.46.0 >
diff --git a/include/linux/hrtimer_defs.h b/include/linux/hrtimer_defs.h index c3b4b7ed7c16..84a5045f80f3 100644 --- a/include/linux/hrtimer_defs.h +++ b/include/linux/hrtimer_defs.h @@ -125,6 +125,7 @@ struct hrtimer_cpu_base { ktime_t softirq_expires_next; struct hrtimer *softirq_next_timer; struct hrtimer_clock_base clock_base[HRTIMER_MAX_CLOCK_BASES]; + call_single_data_t csd; } ____cacheline_aligned; diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c index 030426c8c944..082d4b687fa1 100644 --- a/kernel/time/hrtimer.c +++ b/kernel/time/hrtimer.c @@ -58,6 +58,8 @@ #define HRTIMER_ACTIVE_SOFT (HRTIMER_ACTIVE_HARD << MASK_SHIFT) #define HRTIMER_ACTIVE_ALL (HRTIMER_ACTIVE_SOFT | HRTIMER_ACTIVE_HARD) +static void retrigger_next_event(void *arg); + /* * The timer bases: * @@ -111,7 +113,8 @@ DEFINE_PER_CPU(struct hrtimer_cpu_base, hrtimer_bases) = .clockid = CLOCK_TAI, .get_time = &ktime_get_clocktai, }, - } + }, + .csd = CSD_INIT(retrigger_next_event, NULL) }; static const int hrtimer_clock_to_base_table[MAX_CLOCKS] = { @@ -124,6 +127,14 @@ static const int hrtimer_clock_to_base_table[MAX_CLOCKS] = { [CLOCK_TAI] = HRTIMER_BASE_TAI, }; +static inline bool hrtimer_base_is_online(struct hrtimer_cpu_base *base) +{ + if (!IS_ENABLED(CONFIG_HOTPLUG_CPU)) + return true; + else + return likely(base->online); +} + /* * Functions and macros which are different for UP/SMP systems are kept in a * single place @@ -183,27 +194,58 @@ struct hrtimer_clock_base *lock_hrtimer_base(const struct hrtimer *timer, } /* - * We do not migrate the timer when it is expiring before the next - * event on the target cpu. When high resolution is enabled, we cannot - * reprogram the target cpu hardware and we would cause it to fire - * late. To keep it simple, we handle the high resolution enabled and - * disabled case similar. + * Check if the elected target is suitable considering its next + * event and the hotplug state of the current CPU. + * + * If the elected target is remote and its next event is after the timer + * to queue, then a remote reprogram is necessary. However there is no + * guarantee the IPI handling the operation would arrive in time to meet + * the high resolution deadline. In this case the local CPU becomes a + * preferred target, unless it is offline. + * + * High and low resolution modes are handled the same way for simplicity. * * Called with cpu_base->lock of target cpu held. */ -static int -hrtimer_check_target(struct hrtimer *timer, struct hrtimer_clock_base *new_base) +static bool +hrtimer_suitable_target(struct hrtimer *timer, + struct hrtimer_clock_base *new_base, + struct hrtimer_cpu_base *new_cpu_base, + struct hrtimer_cpu_base *this_cpu_base) { ktime_t expires; + /* + * The local CPU clockevent can be reprogrammed. Also get_target_base() + * guarantees it is online. + */ + if (new_cpu_base == this_cpu_base) + return true; + + /* + * The offline local CPU can't be the default target if the + * next remote target event is after this timer. Keep the + * elected new base. An IPI will we issued to reprogram + * it as a last resort. + */ + if (!hrtimer_base_is_online(this_cpu_base)) + return true; + expires = ktime_sub(hrtimer_get_expires(timer), new_base->offset); - return expires < new_base->cpu_base->expires_next; + + return expires >= new_base->cpu_base->expires_next; } static inline struct hrtimer_cpu_base *get_target_base(struct hrtimer_cpu_base *base, int pinned) { + if (!hrtimer_base_is_online(base)) { + int cpu = cpumask_any_and(cpu_online_mask, housekeeping_cpumask(HK_TYPE_TIMER)); + + return &per_cpu(hrtimer_bases, cpu); + } + #if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON) if (static_branch_likely(&timers_migration_enabled) && !pinned) return &per_cpu(hrtimer_bases, get_nohz_timer_target()); @@ -254,8 +296,8 @@ switch_hrtimer_base(struct hrtimer *timer, struct hrtimer_clock_base *base, raw_spin_unlock(&base->cpu_base->lock); raw_spin_lock(&new_base->cpu_base->lock); - if (new_cpu_base != this_cpu_base && - hrtimer_check_target(timer, new_base)) { + if (!hrtimer_suitable_target(timer, new_base, new_cpu_base, + this_cpu_base)) { raw_spin_unlock(&new_base->cpu_base->lock); raw_spin_lock(&base->cpu_base->lock); new_cpu_base = this_cpu_base; @@ -264,8 +306,8 @@ switch_hrtimer_base(struct hrtimer *timer, struct hrtimer_clock_base *base, } WRITE_ONCE(timer->base, new_base); } else { - if (new_cpu_base != this_cpu_base && - hrtimer_check_target(timer, new_base)) { + if (!hrtimer_suitable_target(timer, new_base, new_cpu_base, + this_cpu_base)) { new_cpu_base = this_cpu_base; goto again; } @@ -716,8 +758,6 @@ static inline int hrtimer_is_hres_enabled(void) return hrtimer_hres_enabled; } -static void retrigger_next_event(void *arg); - /* * Switch to high resolution mode */ @@ -1206,6 +1246,7 @@ static int __hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim, u64 delta_ns, const enum hrtimer_mode mode, struct hrtimer_clock_base *base) { + struct hrtimer_cpu_base *this_cpu_base = this_cpu_ptr(&hrtimer_bases); struct hrtimer_clock_base *new_base; bool force_local, first; @@ -1217,9 +1258,15 @@ static int __hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim, * and enforce reprogramming after it is queued no matter whether * it is the new first expiring timer again or not. */ - force_local = base->cpu_base == this_cpu_ptr(&hrtimer_bases); + force_local = base->cpu_base == this_cpu_base; force_local &= base->cpu_base->next_timer == timer; + /* + * Don't force local queuing if this enqueue happens on a unplugged + * CPU after hrtimer_cpu_dying() has been invoked. + */ + force_local &= this_cpu_base->online; + /* * Remove an active timer from the queue. In case it is not queued * on the current CPU, make sure that remove_hrtimer() updates the @@ -1249,8 +1296,17 @@ static int __hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim, } first = enqueue_hrtimer(timer, new_base, mode); - if (!force_local) - return first; + if (!force_local) { + if (hrtimer_base_is_online(this_cpu_base)) + return first; + + if (first) { + struct hrtimer_cpu_base *new_cpu_base = new_base->cpu_base; + + smp_call_function_single_async(new_cpu_base->cpu, &new_cpu_base->csd); + } + return 0; + } /* * Timer was forced to stay on the current CPU to avoid
hrtimers are migrated away from the dying CPU to any online target at the CPUHP_AP_HRTIMERS_DYING stage in order not to delay bandwidth timers handling tasks involved in the CPU hotplug forward progress. However wake ups can still be performed by the outgoing CPU after CPUHP_AP_HRTIMERS_DYING. Those can result again in bandwidth timers being armed. Depending on several considerations (crystal ball power management based election, earliest timer already enqueued, timer migration enabled or not), the target may eventually be the current CPU even if offline. If that happens, the timer is eventually ignored. The most notable example is RCU which had to deal with each and every of those wake-ups by deferring them to an online CPU, along with related workarounds: _ e787644caf76 (rcu: Defer RCU kthreads wakeup when CPU is dying) _ 9139f93209d1 (rcu/nocb: Fix RT throttling hrtimer armed from offline CPU) _ f7345ccc62a4 (rcu/nocb: Fix rcuog wake-up from offline softirq) The problem isn't confined to RCU though as the stop machine kthread (which runs CPUHP_AP_HRTIMERS_DYING) reports its completion at the end of its work through cpu_stop_signal_done() and performs a wake up that eventually arms the deadline server timer: WARNING: CPU: 94 PID: 588 at kernel/time/hrtimer.c:1086 hrtimer_start_range_ns+0x289/0x2d0 CPU: 94 UID: 0 PID: 588 Comm: migration/94 Not tainted Stopper: multi_cpu_stop+0x0/0x120 <- stop_machine_cpuslocked+0x66/0xc0 RIP: 0010:hrtimer_start_range_ns+0x289/0x2d0 Call Trace: <TASK> ? hrtimer_start_range_ns start_dl_timer enqueue_dl_entity dl_server_start enqueue_task_fair enqueue_task ttwu_do_activate try_to_wake_up complete cpu_stopper_thread smpboot_thread_fn kthread ret_from_fork ret_from_fork_asm </TASK> Instead of providing yet another bandaid to work around the situation, fix it from hrtimers infrastructure instead: always migrate away a timer to an online target whenever it is enqueued from an offline CPU. This will also allow to revert all the above RCU disgraceful hacks. Reported-by: Vlad Poenaru <vlad.wing@gmail.com> Reported-by: Usama Arif <usamaarif642@gmail.com> Fixes: 5c0930ccaad5 ("hrtimers: Push pending hrtimers away from outgoing CPU earlier") Closes: 20241213203739.1519801-1-usamaarif642@gmail.com Signed-off-by: Frederic Weisbecker <frederic@kernel.org> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> --- include/linux/hrtimer_defs.h | 1 + kernel/time/hrtimer.c | 92 +++++++++++++++++++++++++++++------- 2 files changed, 75 insertions(+), 18 deletions(-)