Re: [PATCH v2 2/2] perf/x86/amd: Stop calling amd_pmu_cpu_reset() from amd_pmu_cpu_dead()

[Thomas Gleixner]

On Thu, Oct 26 2023 at 12:03, Mario Limonciello wrote:

> During suspend testing on a workstation CPU a preemption BUG was
> reported.

How is this related to a workstation CPU? Laptop CPUs and server CPUs
are magically not affected, right?

Also how is this related to suspend?

This clearly affects any CPU down operation whether in the context of
suspend or initiated via sysfs, no?

Just because you observed it during suspend testing does not magically
make it a suspend related problem....

> BUG: using smp_processor_id() in preemptible [00000000] code: rtcwake/2960
> caller is amd_pmu_lbr_reset+0x19/0xc0
> CPU: 104 PID: 2960 Comm: rtcwake Not tainted 6.6.0-rc6-00002-g3e2c7f3ac51f
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0x44/0x60
>  check_preemption_disabled+0xce/0xf0
>  ? __pfx_x86_pmu_dead_cpu+0x10/0x10
>  amd_pmu_lbr_reset+0x19/0xc0
>  ? __pfx_x86_pmu_dead_cpu+0x10/0x10
>  amd_pmu_cpu_reset.constprop.0+0x51/0x60
>  amd_pmu_cpu_dead+0x3e/0x90
>  x86_pmu_dead_cpu+0x13/0x20
>  cpuhp_invoke_callback+0x169/0x4b0
>  ? __pfx_virtnet_cpu_dead+0x10/0x10
>  __cpuhp_invoke_callback_range+0x76/0xe0
>  _cpu_down+0x112/0x270
>  freeze_secondary_cpus+0x8e/0x280
>  suspend_devices_and_enter+0x342/0x900
>  pm_suspend+0x2fd/0x690
>  state_store+0x71/0xd0
>  kernfs_fop_write_iter+0x128/0x1c0
>  vfs_write+0x2db/0x400
>  ksys_write+0x5f/0xe0
>  do_syscall_64+0x59/0x90
>  ? srso_alias_return_thunk+0x5/0x7f
>  ? count_memcg_events.constprop.0+0x1a/0x30
>  ? srso_alias_return_thunk+0x5/0x7f
>  ? handle_mm_fault+0x1e9/0x340
>  ? srso_alias_return_thunk+0x5/0x7f
>  ? preempt_count_add+0x4d/0xa0
>  ? srso_alias_return_thunk+0x5/0x7f
>  ? up_read+0x38/0x70
>  ? srso_alias_return_thunk+0x5/0x7f
>  ? do_user_addr_fault+0x343/0x6b0
>  ? srso_alias_return_thunk+0x5/0x7f
>  ? exc_page_fault+0x74/0x170
>  entry_SYSCALL_64_after_hwframe+0x6e/0xd8
> RIP: 0033:0x7f32f8d14a77
> Code: 10 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 00 f3 0f 1e fa
> 64 8b 04 25 18 00 00 00 85 c0 75 10 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff
> 77 51 c3 48 83 ec 28 48 89 54 24 18 48 89 74 24
> RSP: 002b:00007ffdc648de18 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
> RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 00007f32f8d14a77
> RDX: 0000000000000004 RSI: 000055b2fc2a5670 RDI: 0000000000000004
> RBP: 000055b2fc2a5670 R08: 0000000000000000 R09: 000055b2fc2a5670
> R10: 00007f32f8e1a2f0 R11: 0000000000000246 R12: 0000000000000004
> R13: 000055b2fc2a2480 R14: 00007f32f8e16600 R15: 00007f32f8e15a00
>  </TASK>

How much of that backtrace is actually substantial information?

At max 5 lines out of ~50. See:

  https://www.kernel.org/doc/html/latest/process/submitting-patches.html#backtraces

> This bug shows that there is a mistake with the flow used for offlining

This bug shows nothing than a calltrace. Please explain the context and
the fail in coherent sentences. The bug backtrace is just for
illustration.

> a CPU.  Calling amd_pmu_cpu_reset() from the dead callback is
> problematic

It's not problematic. It's simply wrong.

> because this doesn't run on the actual CPU being offlined.  The intent of
> the function is to reset MSRs local to that CPU.
>
> Move the call into the dying callback which is actually run on the local
> CPU.

...

> +static void amd_pmu_cpu_dying(int cpu)
> +{
> +	amd_pmu_cpu_reset(cpu);
> +}

You clearly can spare that wrapper which wraps a function with the signature

     void fn(int)

into a function with the signature

     void fn(int)

by just assigning amd_pmu_cpu_reset() to the cpu_dying callback, no?

Thanks,

        tglx