Re: [PATCH] cpufreq: intel_pstate: Fix for HWP interrupt before driver is ready

[Srinivas Pandruvada]

On Mon, 2021-09-06 at 18:17 +0200, Rafael J. Wysocki wrote:
> On Sat, Sep 4, 2021 at 7:37 AM Srinivas Pandruvada
> <srinivas.pandruvada@xxxxxxxxxxxxxxx> wrote:
> > 
> > In Lenovo X1 gen9 laptop, HWP interrupts arrive before driver is
> > ready
> > to handle on that CPU. Basically didn't even allocated memory for
> > per
> > cpu data structure and not even started interrupt enable process on
> > that
> > CPU. So interrupt handler observes a NULL pointer to schedule work.
> > 
> > This interrupt was probably for SMM, but since it is redirected to
> > OS by OSC call, OS receives it, but not ready to handle. That
> > redirection
> > of interrupt to OS was also done to solve one SMM crash on Yoga 260
> > for
> > HWP interrupt a while back.
> > 
> > To solve this the HWP interrupt handler should ignore such request
> > if the
> > driver is not ready. This will require some flag to wait till the
> > driver
> > setup a workqueue to handle on a CPU. We can't simply assume
> > cpudata to
> > be NULL and avoid processing as it may not be NULL but data
> > structure is
> > not in consistent state.
> > 
> > So created a cpumask which sets the CPU on which interrupt was
> > setup. If
> > not setup, simply clear the interrupt status and return. Since the
> > similar issue can happen during S3 resume, clear the bit during
> > offline.
> > 
> > Since interrupt timing may be before HWP is enabled, use safe MSR
> > read
> > writes as before the change for HWP interrupt.
> > 
> > Fixes: d0e936adbd22 ("cpufreq: intel_pstate: Process HWP Guaranteed
> > change notification")
> > Reported-and-tested-by: Jens Axboe <axboe@xxxxxxxxx>
> > Signed-off-by: Srinivas Pandruvada <   
> > srinivas.pandruvada@xxxxxxxxxxxxxxx>
> > ---
> >  drivers/cpufreq/intel_pstate.c | 23 ++++++++++++++++++++++-
> >  1 file changed, 22 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/cpufreq/intel_pstate.c
> > b/drivers/cpufreq/intel_pstate.c
> > index b4ffe6c8a0d0..5ac86bfa1080 100644
> > --- a/drivers/cpufreq/intel_pstate.c
> > +++ b/drivers/cpufreq/intel_pstate.c
> > @@ -298,6 +298,8 @@ static bool hwp_boost __read_mostly;
> > 
> >  static struct cpufreq_driver *intel_pstate_driver __read_mostly;
> > 
> > +static cpumask_t hwp_intr_enable_mask;
> > +
> >  #ifdef CONFIG_ACPI
> >  static bool acpi_ppc;
> >  #endif
> > @@ -1067,11 +1069,15 @@ static void intel_pstate_hwp_set(unsigned
> > int cpu)
> >         wrmsrl_on_cpu(cpu, MSR_HWP_REQUEST, value);
> >  }
> > 
> > +static void intel_pstate_disable_hwp_interrupt(struct cpudata
> > *cpudata);
> > +
> >  static void intel_pstate_hwp_offline(struct cpudata *cpu)
> >  {
> >         u64 value = READ_ONCE(cpu->hwp_req_cached);
> >         int min_perf;
> > 
> > +       intel_pstate_disable_hwp_interrupt(cpu);
> > +
> >         if (boot_cpu_has(X86_FEATURE_HWP_EPP)) {
> >                 /*
> >                  * In case the EPP has been set to "performance" by
> > the
> > @@ -1645,20 +1651,35 @@ void notify_hwp_interrupt(void)
> >         if (!hwp_active || !boot_cpu_has(X86_FEATURE_HWP_NOTIFY))
> >                 return;
> > 
> > -       rdmsrl(MSR_HWP_STATUS, value);
> > +       rdmsrl_safe(MSR_HWP_STATUS, &value);
> >         if (!(value & 0x01))
> >                 return;
> > 
> > +       if (!cpumask_test_cpu(this_cpu, &hwp_intr_enable_mask)) {
> > +               wrmsrl_safe(MSR_HWP_STATUS, 0);
> > +               return;
> > +       }
> 
> Without additional locking, there is a race between this and
> intel_pstate_disable_hwp_interrupt().
> 
Let's check.

> 1. notify_hwp_interrupt() checks hwp_intr_enable_mask() and the
> target
> CPU is in there, so it will go for scheduling the delayed work.


> 2. intel_pstate_disable_hwp_interrupt() runs between the check and
> the
> cpudata load below.

You mean it didn't call schedule_delayed_work_on(). We are running on
top half of the interrupt handler here, which will not be preempted on
this CPU. intel_pstate_disable_hwp_interrupt() runs on a task context,
which can't preempt ISR. 



> 3. hwp_notify_work is scheduled on the CPU that isn't there in the
> mask any more.

Not sure how top half can be preempted, which can only be preempted
from higher priority interrupt only, but offline callbacks are not
running as ISRs. 

If intel_pstate_disable_hwp_interrupt() started before and interrupt
arrives, ISR will start running. If CPU is removed from mask it ISR
will not do anything, if it arrives before ISR will run to completion. 

Thanks,
Srinivas


> 
> I think that this should be avoided?
> 
> > +
> >         cpudata = all_cpu_data[this_cpu];
> >         schedule_delayed_work_on(this_cpu, &cpudata-
> > >hwp_notify_work, msecs_to_jiffies(10));
> >  }
> > 
> > +static void intel_pstate_disable_hwp_interrupt(struct cpudata
> > *cpudata)
> > +{
> > +
> > +       if (cpumask_test_and_clear_cpu(cpudata->cpu,
> > &hwp_intr_enable_mask)) {
> > +               wrmsrl_on_cpu(cpudata->cpu, MSR_HWP_INTERRUPT,
> > 0x00);
> > +               cancel_delayed_work_sync(&cpudata-
> > >hwp_notify_work);
> > +       }
> > +}
> > +
> >  static void intel_pstate_enable_hwp_interrupt(struct cpudata
> > *cpudata)
> >  {
> >         /* Enable HWP notification interrupt for guaranteed
> > performance change */
> >         if (boot_cpu_has(X86_FEATURE_HWP_NOTIFY)) {
> >                 INIT_DELAYED_WORK(&cpudata->hwp_notify_work,
> > intel_pstate_notify_work);
> >                 wrmsrl_on_cpu(cpudata->cpu, MSR_HWP_INTERRUPT,
> > 0x01);
> > +               cpumask_set_cpu(cpudata->cpu,
> > &hwp_intr_enable_mask);
> >         }
> >  }
> > 
> > --
> > 2.31.1
> >