| Message ID | 20241218165008.37820-2-frederic@kernel.org (mailing list archive) |
|---|---|
| State | Superseded |
| Commit | fe2e53971593e84616d31630458d80398e18fbcf |
| Headers | show |
| Series | hrtimer: Fix timers queued locally from offline CPUs | expand |
On 18/12/2024 19:50, 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 an 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 > 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 > Modules linked in: > 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 > Code: 41 5c 41 5d 41 5e 41 5f 5d e9 63 94 ea 00 0f 0b 48 83 c4 10 5b 41 5c 41 5d 41 5e 41 5f 5d e9 39 fc 15 01 0f 0b e9 c1 fd ff ff <0f> 0b 48 8b > +45 00 e9 59 ff ff ff f3 0f 1e fa 65 8b 05 1d ec e8 7e > RSP: 0018:ffffc900019cbcc8 EFLAGS: 00010046 > RAX: ffff88bf449a4c40 RBX: 0000000000000082 RCX: 0000000000000001 > RDX: 0000000000000001 RSI: ffff88bf43224c80 RDI: ffff88bf449a4c40 > RBP: ffff88bf449a4c80 R08: ffff888280970090 R09: 0000000000000000 > R10: ffff88bf432252e0 R11: ffffffff811abf70 R12: ffff88bf449a4c40 > R13: ffff88bf43234b28 R14: ffff88bf43224c80 R15: 0000000000000000 > FS: 0000000000000000(0000) GS:ffff88bf44980000(0000) knlGS:0000000000000000 > CR2: 0000000000000000 CR3: 000000404b230001 CR4: 0000000000770ef0 > PKRU: 55555554 > Call Trace: > <TASK> > ? __warn+0xcf/0x1b0 > ? hrtimer_start_range_ns+0x289/0x2d0 > ? report_bug+0x120/0x1a0 > ? handle_bug+0x60/0x90 > ? exc_invalid_op+0x1a/0x50 > ? asm_exc_invalid_op+0x1a/0x20 > ? register_refined_jiffies+0xb0/0xb0 > ? hrtimer_start_range_ns+0x289/0x2d0 > ? hrtimer_start_range_ns+0x186/0x2d0 > start_dl_timer+0xfc/0x150 > enqueue_dl_entity+0x367/0x640 > dl_server_start+0x53/0xa0 > enqueue_task_fair+0x363/0x460 > enqueue_task+0x3c/0x200 > ttwu_do_activate+0x94/0x240 > try_to_wake_up+0x315/0x600 > complete+0x4b/0x80 > > 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 us to revert all the above RCU disgraceful hacks. > > Reported-by: 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> > --- > include/linux/hrtimer_defs.h | 1 + > kernel/time/hrtimer.c | 60 +++++++++++++++++++++++++++++++----- > 2 files changed, 54 insertions(+), 7 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 80fe3749d2db..48c0078d2c4f 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] = { > @@ -716,8 +719,6 @@ static inline int hrtimer_is_hres_enabled(void) > return hrtimer_hres_enabled; > } > > -static void retrigger_next_event(void *arg); > - > /* > * Switch to high resolution mode > */ > @@ -761,6 +762,8 @@ static void retrigger_next_event(void *arg) > { > struct hrtimer_cpu_base *base = this_cpu_ptr(&hrtimer_bases); > > + guard(raw_spinlock)(&base->lock); > + > /* > * When high resolution mode or nohz is active, then the offsets of > * CLOCK_REALTIME/TAI/BOOTTIME have to be updated. Otherwise the > @@ -773,18 +776,17 @@ static void retrigger_next_event(void *arg) > * In the NOHZ case the update of the offset and the reevaluation > * of the next expiring timer is enough. The return from the SMP > * function call will take care of the reprogramming in case the > - * CPU was in a NOHZ idle sleep. > + * CPU was in a NOHZ idle sleep. base->lock must still be held to > + * to release and synchronize against the csd unlock. > */ > if (!hrtimer_hres_active(base) && !tick_nohz_active) > return; > > - raw_spin_lock(&base->lock); > hrtimer_update_base(base); > if (hrtimer_hres_active(base)) > hrtimer_force_reprogram(base, 0); > else > hrtimer_update_next_event(base); > - raw_spin_unlock(&base->lock); > } > > /* > @@ -1202,10 +1204,48 @@ hrtimer_update_softirq_timer(struct hrtimer_cpu_base *cpu_base, bool reprogram) > hrtimer_reprogram(cpu_base->softirq_next_timer, reprogram); > } > > +/* > + * If the current CPU is offline and timers have been already > + * migrated away, make sure not to enqueue locally and perform > + * a remote retrigger as a last resort. > + */ > +static void enqueue_hrtimer_offline(struct hrtimer *timer, > + struct hrtimer_clock_base *base, > + const enum hrtimer_mode mode) > +{ > + struct hrtimer_cpu_base *new_cpu_base, *old_cpu_base, *this_cpu_base; > + struct hrtimer_clock_base *new_base; > + int cpu; > + > + old_cpu_base = base->cpu_base; > + this_cpu_base = this_cpu_ptr(&hrtimer_bases); > + > + if (old_cpu_base == this_cpu_base || !old_cpu_base->online) { > + WARN_ON_ONCE(hrtimer_callback_running(timer)); > + cpu = cpumask_any_and(cpu_online_mask, > + housekeeping_cpumask(HK_TYPE_TIMER)); > + new_cpu_base = &per_cpu(hrtimer_bases, cpu); > + new_base = &new_cpu_base->clock_base[base->index]; > + WRITE_ONCE(timer->base, &migration_base); > + raw_spin_unlock(&old_cpu_base->lock); > + raw_spin_lock(&new_cpu_base->lock); > + WRITE_ONCE(timer->base, new_base); > + } else { > + new_base = base; > + new_cpu_base = new_base->cpu_base; > + cpu = new_cpu_base->cpu; > + } > + > + if (enqueue_hrtimer(timer, new_base, mode)) > + smp_call_function_single_async(cpu, &new_cpu_base->csd); > +} > + > + > 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; > > @@ -1217,7 +1257,7 @@ 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; > > /* > @@ -1240,6 +1280,12 @@ static int __hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim, > > hrtimer_set_expires_range_ns(timer, tim, delta_ns); > > + if (unlikely(!this_cpu_base->online)) { > + enqueue_hrtimer_offline(timer, base, mode); Thanks for the fix! It looks good to me, maybe as a follow up, we could rename switch_hrtimer_base to enqueue_hrtimer_local? (or maybe something more appropriate) There are now 2 different paths that switch hrtimer base (enqueue_hrtimer_offline and switch_hrtimer_base). > + return 0; > + } > + > + nit: extra new line above. > /* Switch the timer base, if necessary: */ > if (!force_local) { > new_base = switch_hrtimer_base(timer, base,
Le Thu, Dec 19, 2024 at 10:00:12PM +0300, Usama Arif a écrit : > > @@ -1240,6 +1280,12 @@ static int __hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim, > > > > hrtimer_set_expires_range_ns(timer, tim, delta_ns); > > > > + if (unlikely(!this_cpu_base->online)) { > > + enqueue_hrtimer_offline(timer, base, mode); > > Thanks for the fix! > > It looks good to me, maybe as a follow up, we could rename switch_hrtimer_base to > enqueue_hrtimer_local? (or maybe something more appropriate) > There are now 2 different paths that switch hrtimer base (enqueue_hrtimer_offline and > switch_hrtimer_base). I considered extending switch_hrtimer_base() instead to handle offline CPU from there but that turned out ugly since what follows assumes to either queue locally and possibly reprogram or queue remotely and not reprogram. enqueue_hrtimer_global() does only enqueue remotely and possibly trigger a reprogram. And indeed we could move switch_hrtimer_base() + enqueue_hrtimer() + hrtimer_force_reprogram() to a enqueue_hrtimer_online() for example. Perhaps that would clarify things a bit, I don't know... Thanks. > > > + return 0; > > + } > > + > > + > nit: extra new line above. > > /* Switch the timer base, if necessary: */ > > if (!force_local) { > > new_base = switch_hrtimer_base(timer, base, >
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 80fe3749d2db..48c0078d2c4f 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] = { @@ -716,8 +719,6 @@ static inline int hrtimer_is_hres_enabled(void) return hrtimer_hres_enabled; } -static void retrigger_next_event(void *arg); - /* * Switch to high resolution mode */ @@ -761,6 +762,8 @@ static void retrigger_next_event(void *arg) { struct hrtimer_cpu_base *base = this_cpu_ptr(&hrtimer_bases); + guard(raw_spinlock)(&base->lock); + /* * When high resolution mode or nohz is active, then the offsets of * CLOCK_REALTIME/TAI/BOOTTIME have to be updated. Otherwise the @@ -773,18 +776,17 @@ static void retrigger_next_event(void *arg) * In the NOHZ case the update of the offset and the reevaluation * of the next expiring timer is enough. The return from the SMP * function call will take care of the reprogramming in case the - * CPU was in a NOHZ idle sleep. + * CPU was in a NOHZ idle sleep. base->lock must still be held to + * to release and synchronize against the csd unlock. */ if (!hrtimer_hres_active(base) && !tick_nohz_active) return; - raw_spin_lock(&base->lock); hrtimer_update_base(base); if (hrtimer_hres_active(base)) hrtimer_force_reprogram(base, 0); else hrtimer_update_next_event(base); - raw_spin_unlock(&base->lock); } /* @@ -1202,10 +1204,48 @@ hrtimer_update_softirq_timer(struct hrtimer_cpu_base *cpu_base, bool reprogram) hrtimer_reprogram(cpu_base->softirq_next_timer, reprogram); } +/* + * If the current CPU is offline and timers have been already + * migrated away, make sure not to enqueue locally and perform + * a remote retrigger as a last resort. + */ +static void enqueue_hrtimer_offline(struct hrtimer *timer, + struct hrtimer_clock_base *base, + const enum hrtimer_mode mode) +{ + struct hrtimer_cpu_base *new_cpu_base, *old_cpu_base, *this_cpu_base; + struct hrtimer_clock_base *new_base; + int cpu; + + old_cpu_base = base->cpu_base; + this_cpu_base = this_cpu_ptr(&hrtimer_bases); + + if (old_cpu_base == this_cpu_base || !old_cpu_base->online) { + WARN_ON_ONCE(hrtimer_callback_running(timer)); + cpu = cpumask_any_and(cpu_online_mask, + housekeeping_cpumask(HK_TYPE_TIMER)); + new_cpu_base = &per_cpu(hrtimer_bases, cpu); + new_base = &new_cpu_base->clock_base[base->index]; + WRITE_ONCE(timer->base, &migration_base); + raw_spin_unlock(&old_cpu_base->lock); + raw_spin_lock(&new_cpu_base->lock); + WRITE_ONCE(timer->base, new_base); + } else { + new_base = base; + new_cpu_base = new_base->cpu_base; + cpu = new_cpu_base->cpu; + } + + if (enqueue_hrtimer(timer, new_base, mode)) + smp_call_function_single_async(cpu, &new_cpu_base->csd); +} + + 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; @@ -1217,7 +1257,7 @@ 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; /* @@ -1240,6 +1280,12 @@ static int __hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim, hrtimer_set_expires_range_ns(timer, tim, delta_ns); + if (unlikely(!this_cpu_base->online)) { + enqueue_hrtimer_offline(timer, base, mode); + return 0; + } + + /* Switch the timer base, if necessary: */ if (!force_local) { new_base = switch_hrtimer_base(timer, base,
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 an 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 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 Modules linked in: 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 Code: 41 5c 41 5d 41 5e 41 5f 5d e9 63 94 ea 00 0f 0b 48 83 c4 10 5b 41 5c 41 5d 41 5e 41 5f 5d e9 39 fc 15 01 0f 0b e9 c1 fd ff ff <0f> 0b 48 8b +45 00 e9 59 ff ff ff f3 0f 1e fa 65 8b 05 1d ec e8 7e RSP: 0018:ffffc900019cbcc8 EFLAGS: 00010046 RAX: ffff88bf449a4c40 RBX: 0000000000000082 RCX: 0000000000000001 RDX: 0000000000000001 RSI: ffff88bf43224c80 RDI: ffff88bf449a4c40 RBP: ffff88bf449a4c80 R08: ffff888280970090 R09: 0000000000000000 R10: ffff88bf432252e0 R11: ffffffff811abf70 R12: ffff88bf449a4c40 R13: ffff88bf43234b28 R14: ffff88bf43224c80 R15: 0000000000000000 FS: 0000000000000000(0000) GS:ffff88bf44980000(0000) knlGS:0000000000000000 CR2: 0000000000000000 CR3: 000000404b230001 CR4: 0000000000770ef0 PKRU: 55555554 Call Trace: <TASK> ? __warn+0xcf/0x1b0 ? hrtimer_start_range_ns+0x289/0x2d0 ? report_bug+0x120/0x1a0 ? handle_bug+0x60/0x90 ? exc_invalid_op+0x1a/0x50 ? asm_exc_invalid_op+0x1a/0x20 ? register_refined_jiffies+0xb0/0xb0 ? hrtimer_start_range_ns+0x289/0x2d0 ? hrtimer_start_range_ns+0x186/0x2d0 start_dl_timer+0xfc/0x150 enqueue_dl_entity+0x367/0x640 dl_server_start+0x53/0xa0 enqueue_task_fair+0x363/0x460 enqueue_task+0x3c/0x200 ttwu_do_activate+0x94/0x240 try_to_wake_up+0x315/0x600 complete+0x4b/0x80 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 us to revert all the above RCU disgraceful hacks. Reported-by: 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> --- include/linux/hrtimer_defs.h | 1 + kernel/time/hrtimer.c | 60 +++++++++++++++++++++++++++++++----- 2 files changed, 54 insertions(+), 7 deletions(-)