Re: [PATCH V3] perf: Fix the throttle error of some clock events

["Liang, Kan" <kan.liang@linux.intel.com>]

On 2025-06-05 2:45 p.m., Alexei Starovoitov wrote:
> On Thu, Jun 5, 2025 at 6:46 AM Liang, Kan <kan.liang@linux.intel.com> wrote:
>>
>>
>>
>> On 2025-06-04 7:21 p.m., Alexei Starovoitov wrote:
>>> On Wed, Jun 4, 2025 at 10:16 AM <kan.liang@linux.intel.com> wrote:
>>>>
>>>> From: Kan Liang <kan.liang@linux.intel.com>
>>>>
>>>> Both ARM and IBM CI reports RCU stall, which can be reproduced by the
>>>> below perf command.
>>>>   perf record -a -e cpu-clock -- sleep 2
>>>>
>>>> The issue is introduced by the generic throttle patch set, which
>>>> unconditionally invoke the event_stop() when throttle is triggered.
>>>>
>>>> The cpu-clock and task-clock are two special SW events, which rely on
>>>> the hrtimer. The throttle is invoked in the hrtimer handler. The
>>>> event_stop()->hrtimer_cancel() waits for the handler to finish, which is
>>>> a deadlock. Instead of invoking the stop(), the HRTIMER_NORESTART should
>>>> be used to stop the timer.
>>>>
>>>> There may be two ways to fix it.
>>>> - Introduce a PMU flag to track the case. Avoid the event_stop in
>>>>   perf_event_throttle() if the flag is detected.
>>>>   It has been implemented in the
>>>>   https://lore.kernel.org/lkml/20250528175832.2999139-1-kan.liang@linux.intel.com/
>>>>   The new flag was thought to be an overkill for the issue.
>>>> - Add a check in the event_stop. Return immediately if the throttle is
>>>>   invoked in the hrtimer handler. Rely on the existing HRTIMER_NORESTART
>>>>   method to stop the timer.
>>>>
>>>> The latter is implemented here.
>>>>
>>>> Move event->hw.interrupts = MAX_INTERRUPTS before the stop(). It makes
>>>> the order the same as perf_event_unthrottle(). Except the patch, no one
>>>> checks the hw.interrupts in the stop(). There is no impact from the
>>>> order change.
>>>>
>>>> Reported-by: Leo Yan <leo.yan@arm.com>
>>>> Reported-by: Aishwarya TCV <aishwarya.tcv@arm.com>
>>>> Closes: https://lore.kernel.org/lkml/20250527161656.GJ2566836@e132581.arm.com/
>>>> Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
>>>> Closes: https://lore.kernel.org/lkml/djxlh5fx326gcenwrr52ry3pk4wxmugu4jccdjysza7tlc5fef@ktp4rffawgcw/
>>>> Reported-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
>>>> Closes: https://lore.kernel.org/lkml/8e8f51d8-af64-4d9e-934b-c0ee9f131293@linux.ibm.com/
>>>> Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
>>>
>>> It seems the patch fixes one issue and introduces another ?
>>>
>>> Looks like the throttle event is sticky.
>>> Once it's reached the perf_event no longer works ?
>>
>> No. It should still work even the throttle is triggered.
>>
>> sdp@d404e6bce080:~$ sudo bash -c 'echo 10 >
>> /proc/sys/kernel/perf_event_max_sample_rate'
>> sdp@d404e6bce080:~$ sudo perf record -a -e cpu-clock -c10000 -- sleep 1
>> [ perf record: Woken up 1 times to write data ]
>> [ perf record: Captured and wrote 0.559 MB perf.data (584 samples) ]
> 
> With the patch applied above command hangs in my VM:
> 
> # perf record -a -e cpu-clock -c10000 -- sleep 1
> [   43.656855] hrtimer: interrupt took 21640 ns
> [   68.561052] watchdog: BUG: soft lockup - CPU#0 stuck for 41s! [perf:2253]
> [   68.561056] Modules linked in: bpf_preload
> [   68.561060] CPU: 0 UID: 0 PID: 2253 Comm: perf Not tainted
> 6.15.0-12294-gc89e5202e569 #1172 PREEMPT
> [   68.561062] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
> BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
> [   68.561063] RIP: 0010:handle_softirqs+0x77/0x2a0
> [   68.561070] Code: 6b 96 02 00 01 00 00 bd 0a 00 00 00 44 89 64 24
> 14 89 6c 24 10 40 88 7c 24 04 31 c0 65 66 89 05 5f 6b 96 02 fb bb ff
> ff ff ff <48> c7 c0 c0 f
> [   68.561071] RSP: 0018:ffa0000000003fa0 EFLAGS: 00000246
> [   68.561072] RAX: 0000000000000000 RBX: 00000000ffffffff RCX: 00000000000006e0
> [   68.561073] RDX: 0000000000000007 RSI: ff1100010212e100 RDI: 0000000000000000
> [   68.561074] RBP: 000000000000000a R08: 0000000000000000 R09: 7fffffffffffffff
> [   68.561074] R10: 00000005a3af2140 R11: 0000000000004601 R12: 0000000000400100
> [   68.561075] R13: 0000000000000000 R14: 0000000000000002 R15: 0000000000000000
> [   68.561111] FS:  00007fa1b536f780(0000) GS:ff110004abe26000(0000)
> knlGS:0000000000000000
> [   68.561112] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   68.561113] CR2: 0000000001c67a30 CR3: 000000010f2f3003 CR4: 00000000003716f0
> [   68.561113] Call Trace:
> [   68.561170]  <IRQ>
> [   68.561174]  irq_exit_rcu+0x91/0xb0
> [   68.561176]  sysvec_apic_timer_interrupt+0x71/0x90
> [   68.561181]  </IRQ>
> [   68.561181]  <TASK>
> [   68.561182]  asm_sysvec_apic_timer_interrupt+0x1a/0x20
> [   68.561184] RIP: 0010:generic_exec_single+0x33/0x120
> [   68.561188] Code: 65 39 3d 5c 4d 89 02 74 28 3b 3d b8 f5 17 02 0f
> 83 de 00 00 00 89 f8 48 0f a3 05 d8 f1 17 02 0f 83 ce 00 00 00 e8 bd
> fe ff ff <31> c0 5b 5d 9
> [   68.561189] RSP: 0018:ffa00000025a3cc0 EFLAGS: 00000206
> [   68.561190] RAX: 0000000000000000 RBX: ffffffff8145b310 RCX: ff110004abe26000
> [   68.561190] RDX: 0000000000000007 RSI: 0000000000000000 RDI: ff1100042fa26540
> [   68.561191] RBP: 0000000000000202 R08: 0000000000000000 R09: 0000000000000000
> [   68.561191] R10: ffa00000025a3ee0 R11: 0000000000000000 R12: ffa00000025a3d40
> [   68.561192] R13: ff11000100881220 R14: ff11000100880fc0 R15: 0000000000000000
> [   68.561192]  ? sw_perf_event_destroy+0x70/0x70
> [   68.561197]  smp_call_function_single+0xc4/0x110
> [   68.561199]  ? sw_perf_event_destroy+0x70/0x70
> [   68.561200]  event_function_call+0x160/0x170
> [   68.561202]  ? ctx_resched+0x2d0/0x2d0
> [   68.561205]  ? perf_event_set_state+0x60/0x60
> [   68.561206]  ? _perf_event_disable+0x50/0x50
> [   68.561208]  perf_event_for_each_child+0x37/0x80
> [   68.561209]  ? _perf_event_disable+0x50/0x50
> [   68.561211]  _perf_ioctl+0x1df/0xad0
> [   68.561213]  ? __set_cpus_allowed_ptr+0x71/0x80
> [   68.561216]  ? avc_has_perm+0x72/0x160
> [   68.561219]  ? ldsem_down_write+0x1bb/0x1fc
> [   68.561222]  perf_ioctl+0x42/0x70
> [   68.561224]  __x64_sys_ioctl+0x8f/0xd0
> [   68.561226]  do_syscall_64+0x46/0x160
> [   68.561228]  entry_SYSCALL_64_after_hwframe+0x4b/0x53
> 
> but only after a fresh boot.
> 
> If I run bpf selftests before that it works.
> Like test_progs -t stacktrace_build_id_nmi
> followed by the same
> perf record -a -e cpu-clock ..
> it works fine.
> 

There should be a bug in V3. When stops in the throttle, the event
should not be updated, stop(event, 0). But the cpu_clock_event_stop()
doesn't handle the flag. That changes the behavior a little bit.

Could you please try the below patch and see if it helps?

diff --git a/kernel/events/core.c b/kernel/events/core.c
index cd85b1820e7d..b6c57ba24e78 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2656,8 +2656,8 @@ static void perf_event_unthrottle(struct
perf_event *event, bool start)

 static void perf_event_throttle(struct perf_event *event)
 {
-	event->pmu->stop(event, 0);
 	event->hw.interrupts = MAX_INTERRUPTS;
+	event->pmu->stop(event, 0);
 	if (event == event->group_leader)
 		perf_log_throttle(event, 0);
 }
@@ -11777,7 +11777,12 @@ static void perf_swevent_cancel_hrtimer(struct
perf_event *event)
 {
 	struct hw_perf_event *hwc = &event->hw;

-	if (is_sampling_event(event)) {
+	/*
+	 * The throttle can be triggered in the hrtimer handler.
+	 * The HRTIMER_NORESTART should be used to stop the timer,
+	 * rather than hrtimer_cancel(). See perf_swevent_hrtimer()
+	 */
+	if (is_sampling_event(event) && (hwc->interrupts != MAX_INTERRUPTS)) {
 		ktime_t remaining = hrtimer_get_remaining(&hwc->hrtimer);
 		local64_set(&hwc->period_left, ktime_to_ns(remaining));

@@ -11832,7 +11837,8 @@ static void cpu_clock_event_start(struct
perf_event *event, int flags)
 static void cpu_clock_event_stop(struct perf_event *event, int flags)
 {
 	perf_swevent_cancel_hrtimer(event);
-	cpu_clock_event_update(event);
+	if (flags & PERF_EF_UPDATE)
+		cpu_clock_event_update(event);
 }

 static int cpu_clock_event_add(struct perf_event *event, int flags)


Thanks,
Kan