Re: [PATCH 2/2] perf/x86/amd: Don't allow pre-emption in amd_pmu_lbr_reset()

[Peter Zijlstra]

On Tue, Oct 24, 2023 at 10:32:27AM -0500, Mario Limonciello wrote:
> On 10/24/2023 03:02, Ingo Molnar wrote:
> > 
> > * Mario Limonciello <mario.limonciello@xxxxxxx> wrote:
> > 
> > > Fixes a BUG reported during suspend to ram testing.
> > > 
> > > ```
> > > [  478.274752] BUG: using smp_processor_id() in preemptible [00000000] code: rtcwake/2948
> > > [  478.274754] caller is amd_pmu_lbr_reset+0x19/0xc0
> > > ```
> > > 
> > > Cc: stable@xxxxxxxxxxxxxxx # 6.1+
> > > Fixes: ca5b7c0d9621 ("perf/x86/amd/lbr: Add LbrExtV2 branch record support")
> > > Signed-off-by: Mario Limonciello <mario.limonciello@xxxxxxx>
> > > ---
> > >   arch/x86/events/amd/lbr.c | 3 ++-
> > >   1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/x86/events/amd/lbr.c b/arch/x86/events/amd/lbr.c
> > > index eb31f850841a..5b98e8c7d8b7 100644
> > > --- a/arch/x86/events/amd/lbr.c
> > > +++ b/arch/x86/events/amd/lbr.c
> > > @@ -321,7 +321,7 @@ int amd_pmu_lbr_hw_config(struct perf_event *event)
> > >   void amd_pmu_lbr_reset(void)
> > >   {
> > > -	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> > > +	struct cpu_hw_events *cpuc = get_cpu_ptr(&cpu_hw_events);
> > >   	int i;
> > >   	if (!x86_pmu.lbr_nr)
> > > @@ -335,6 +335,7 @@ void amd_pmu_lbr_reset(void)
> > >   	cpuc->last_task_ctx = NULL;
> > >   	cpuc->last_log_id = 0;
> > > +	put_cpu_ptr(&cpu_hw_events);
> > >   	wrmsrl(MSR_AMD64_LBR_SELECT, 0);
> > >   }
> > 
> > Weird, amd_pmu_lbr_reset() is called from these places:
> > 
> >    - amd_pmu_lbr_sched_task(): during task sched-in during
> >      context-switching, this should already have preemption disabled.
> > 
> >    - amd_pmu_lbr_add(): this gets indirectly called by amd_pmu::add
> >      (amd_pmu_add_event()), called by event_sched_in(), which too should have
> >      preemption disabled.
> > 
> > I clearly must have missed some additional place it gets called in.
> > 
> > Could you please cite the full log of the amd_pmu_lbr_reset() call that
> > caused the critical section warning?
> > 
> > Thanks,
> > 
> > 	Ingo
> 
> Below is the call trace in case you think it's better to disable preemption
> by the caller instead.  If you think it's better to keep it in
> amd_pmu_lbr_reset() I'll add this trace to the commit message.

You cut too much; what task is running this?

IIRC this is the hotplug thread running a teardown function on that CPU
itself. It being a strict per-cpu thread should not trip
smp_processor_id() wanrs.

> 
> 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
>