[PATCH 1/3 v3] hrtimers: Force migrate away hrtimers queued after CPUHP_AP_HRTIMERS_DYING

Frederic Weisbecker
2024-12-31T17:07:28Z

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 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 | 53 +++++++++++++++++++++++++++++++++---
2 files changed, 50 insertions(+), 4 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..1ea9ef57d710 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
*/
@@ -1202,10 +1203,49 @@ 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)
+{
+#ifdef CONFIG_HOTPLUG_CPU
+ 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);
+#endif
+}
+
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 +1280,11 @@ 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,
--
2.46.0