Date:   Mon, 6 Sep 2021 20:25:57 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Jens Axboe <axboe@...nel.dk>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: intel_pstate: Fix for HWP interrupt before
 driver is ready

On Mon, Sep 6, 2021 at 8:14 PM Srinivas Pandruvada
<srinivas.pandruvada@...ux.intel.com> wrote:
>
> On Mon, 2021-09-06 at 19:54 +0200, Rafael J. Wysocki wrote:
> > On Mon, Sep 6, 2021 at 7:23 PM Srinivas Pandruvada
> > <srinivas.pandruvada@...ux.intel.com> wrote:
> > >
> > > On Mon, 2021-09-06 at 18:58 +0200, Rafael J. Wysocki wrote:
> > > > On Mon, Sep 6, 2021 at 6:55 PM Srinivas Pandruvada
> > > > <srinivas.pandruvada@...ux.intel.com> wrote:
> > > > >
> > > > > On Mon, 2021-09-06 at 10:43 -0600, Jens Axboe wrote:
> > > > > > On 9/6/21 10:17 AM, Rafael J. Wysocki wrote:
> > > > > > > On Sat, Sep 4, 2021 at 7:37 AM Srinivas Pandruvada
> > > > > > > <srinivas.pandruvada@...ux.intel.com> 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@...nel.dk>
> > > > > > > > Signed-off-by: Srinivas Pandruvada <
> > > > > > > > srinivas.pandruvada@...ux.intel.com>
> > > > > > > > ---
> > > > > > > >  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().
> > > > > > >
> > > > > > > 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.
> > > > > > > 3. hwp_notify_work is scheduled on the CPU that isn't there
> > > > > > > in
> > > > > > > the
> > > > > > > mask any more.
> > > > > >
> > > > > > I noticed that too, not clear to me how much of an issue that
> > > > > > is
> > > > > > in
> > > > > > practice. But there's definitely a race there.
> > > > > Glad to see how this is possible from code running in ISR
> > > > > context.
> > > >
> > > > intel_pstate_disable_hwp_interrupt() may very well run on a
> > > > different
> > > > CPU in parallel with the interrupt handler running on this CPU.  Or
> > > > is
> > > > this not possible for some reason?
> > > I see the offline callback is called from cpufreq core from hotplug
> > > online/offline callback. So this should run the call on the target
> > > CPU.
> > > From Documentation
> > > "The states CPUHP_AP_OFFLINE … CPUHP_AP_ONLINE are invoked just the
> > > after the CPU has been brought up. The interrupts are off and the
> > > scheduler is not yet active on this CPU. Starting with
> > > CPUHP_AP_OFFLINE
> > > the callbacks are invoked on the target CPU."
> > >
> > > The only other place it is called is from subsys remove callback. Not
> > > sure how can you remove cpufreq subsys on fly.
> >
> > cpufreq_unregister_driver() causes this to happen.
> >
> > > Let's say it is possible:
> > > While running ISR on a local CPU, how can someone pull the CPU before
> > > its completion? If the CPU is going away after that, the workqueue is
> > > unbounded. So it will run on some other CPU, here if that happens it
> > > will call cpufreq update policy, which will be harmless.
> >
> > Well, it looks to me like if you are super-unlucky, the ISR may run on
> > the local CPU in parallel with intel_pstate_update_status() running on
> > a remote one and so dereferencing cpudata from it is generally unsafe.
> > In theory.  In practice it is unlikely to become problematic for
> > timing reasons AFAICS.
> >
> >
> We are handling offline for other thermal interrupt sources from same
> interrupt in therm-throt.c, where we do similar in offline path (by
> TGLX). If cpufreq offline can cause such issue of changing CPU,

This is not cpufreq offline, but intel_pstate_update_status() which
may be triggered via sysfs.  And again, the theoretically problematic
thing is dereferencing cpudata (which may be cleared by a remote CPU)
from the interrupt handler without protection.

> I can call intel_pstate_disable_hwp_interrupt() via override from
> https://elixir.bootlin.com/linux/latest/C/ident/thermal_throttle_offline
> after masking APIC interrupt.

But why would using RCU be harder than this?

Also please note that on RT kernels interrupt handlers are run in threads.

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