Message-ID: <7638853b-bbc6-464d-8890-29ed92aa5cef@linux.intel.com>
Date: Thu, 5 Jun 2025 19:50:14 -0400
From: "Liang, Kan" <kan.liang@...ux.intel.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Namhyung Kim <namhyung@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Mark Rutland <mark.rutland@....com>, LKML <linux-kernel@...r.kernel.org>,
 "linux-perf-use." <linux-perf-users@...r.kernel.org>,
 Stephane Eranian <eranian@...gle.com>, Chun-Tse Shao <ctshao@...gle.com>,
 Thomas Richter <tmricht@...ux.ibm.com>, Leo Yan <leo.yan@....com>,
 Aishwarya TCV <aishwarya.tcv@....com>,
 Venkat Rao Bagalkote <venkat88@...ux.ibm.com>
Subject: Re: [PATCH V3] perf: Fix the throttle error of some clock events



On 2025-06-05 4:46 p.m., Alexei Starovoitov wrote:
> On Thu, Jun 5, 2025 at 1:24 PM Liang, Kan <kan.liang@...ux.intel.com> wrote:
>>
>>
>>
>> On 2025-06-05 2:45 p.m., Alexei Starovoitov wrote:
>>> On Thu, Jun 5, 2025 at 6:46 AM Liang, Kan <kan.liang@...ux.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@...ux.intel.com> wrote:
>>>>>>
>>>>>> From: Kan Liang <kan.liang@...ux.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@....com>
>>>>>> Reported-by: Aishwarya TCV <aishwarya.tcv@....com>
>>>>>> Closes: https://lore.kernel.org/lkml/20250527161656.GJ2566836@e132581.arm.com/
>>>>>> Reported-by: Alexei Starovoitov <alexei.starovoitov@...il.com>
>>>>>> Closes: https://lore.kernel.org/lkml/djxlh5fx326gcenwrr52ry3pk4wxmugu4jccdjysza7tlc5fef@ktp4rffawgcw/
>>>>>> Reported-by: Venkat Rao Bagalkote <venkat88@...ux.ibm.com>
>>>>>> Closes: https://lore.kernel.org/lkml/8e8f51d8-af64-4d9e-934b-c0ee9f131293@linux.ibm.com/
>>>>>> Signed-off-by: Kan Liang <kan.liang@...ux.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@...4e6bce080:~$ sudo bash -c 'echo 10 >
>>>> /proc/sys/kernel/perf_event_max_sample_rate'
>>>> sdp@...4e6bce080:~$ 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);
>>  }
> 
> Nope. The last hunk didn't make any difference.
> Same soft lockup.

Thanks for the verification.

I have some questions, could you please help to clarify?

- What's the value of proc/sys/kernel/perf_event_max_sample_rate in the
test?

- Can the soft lockup issue be produced by both bpf selftests and the
perf record command? Or just the perf record command?

- Can the soft lockup issue be reproduced with the v2 patchset?
https://lore.kernel.org/lkml/20250528175832.2999139-1-kan.liang@linux.intel.com/

- Furthermore, can the soft lockup issue be reproduced after reverting
the recent generic throttle logic fix?
commit e800ac51202f ("perf: Only dump the throttle log for the leader")
commit 9734e25fbf5a ("perf: Fix the throttle logic for a group")

Thanks,
Kan

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