[PATCH 0/2] More frequency invariance fixes for x86

[PATCH 0/2] More frequency invariance fixes for x86

[Giovanni Gherdovich]

Patch 1/2 prevents a division by zero in case the product
"delta_MPERF * arch_max_freq_ratio" overflows u64, as suggested by Linus at [1].
This patch supersedes the version at [2], as it also disables frequency
invariance when that overflow happens.

Patch 2/2 implements the recommendation by Ricardo Neri to check for an all
zero MSR_TURBO_RATIO_LIMIT and disable freq invariance in that case too.

[1] https://lore.kernel.org/lkml/CAHk-=wiX+NT2yxtdPszH9U_S96MCNQA56GJFXY45mZc47yG5KQ@mail.gmail.com/
[2] https://lore.kernel.org/lkml/20200422144055.18171-1-ggherdovich@suse.cz/
[3] https://lore.kernel.org/lkml/20200424013222.GA26355@ranerica-svr.sc.intel.com/

Giovanni Gherdovich (2):
  x86, sched: Prevent divisions by zero in frequency invariant
    accounting
  x86, sched: Bail out of frequency invariance if turbo frequency is
    unknown

 arch/x86/kernel/smpboot.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

-- 
2.16.4

[PATCH 1/2] x86, sched: Prevent divisions by zero in frequency invariant accounting

[Giovanni Gherdovich]

The product mcnt * arch_max_freq_ratio could be zero if it overflows u64.

For context, a large value for arch_max_freq_ratio would be 5000,
corresponding to a turbo_freq/base_freq ratio of 5 (normally it's more like
1500-2000). A large increment frequency for the MPERF counter would be 5GHz
(the base clock of all CPUs on the market today is less than that). With
these figures, a CPU would need to go without a scheduler tick for around 8
days for the u64 overflow to happen. It is unlikely, but the check is
warranted.

In that case it's also appropriate to disable frequency invariant
accounting: the feature relies on measures of the clock frequency done at
every scheduler tick, which need to be "fresh" to be at all meaningful.

Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")
---
 arch/x86/kernel/smpboot.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 8c89e4d9ad28..4718f29a3065 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -2039,6 +2039,14 @@ static void init_freq_invariance(bool secondary)
 	}
 }
 
+static void disable_freq_invariance_workfn(struct work_struct *work)
+{
+	static_branch_disable(&arch_scale_freq_key);
+}
+
+static DECLARE_WORK(disable_freq_invariance_work,
+		    disable_freq_invariance_workfn);
+
 DEFINE_PER_CPU(unsigned long, arch_freq_scale) = SCHED_CAPACITY_SCALE;
 
 void arch_scale_freq_tick(void)
@@ -2055,14 +2063,18 @@ void arch_scale_freq_tick(void)
 
 	acnt = aperf - this_cpu_read(arch_prev_aperf);
 	mcnt = mperf - this_cpu_read(arch_prev_mperf);
-	if (!mcnt)
-		return;
 
 	this_cpu_write(arch_prev_aperf, aperf);
 	this_cpu_write(arch_prev_mperf, mperf);
 
 	acnt <<= 2*SCHED_CAPACITY_SHIFT;
 	mcnt *= arch_max_freq_ratio;
+	if (!mcnt) {
+		pr_warn("Scheduler tick missing for long time, disabling scale-invariant accounting.\n");
+		/* static_branch_disable() acquires a lock and may sleep */
+		schedule_work(&disable_freq_invariance_work);
+		return;
+	}
 
 	freq_scale = div64_u64(acnt, mcnt);
 
-- 
2.16.4

[PATCH 2/2] x86, sched: Bail out of frequency invariance if turbo frequency is unknown

[Giovanni Gherdovich]

There may be CPUs that support turbo boost but don't declare any turbo
ratio, i.e. their MSR_TURBO_RATIO_LIMIT is all zeroes. In that condition
scale-invariant calculations can't be performed.

Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
Suggested-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")
---
 arch/x86/kernel/smpboot.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 4718f29a3065..ab2a0df7d1fb 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1991,9 +1991,11 @@ static bool intel_set_max_freq_ratio(void)
 	/*
 	 * Some hypervisors advertise X86_FEATURE_APERFMPERF
 	 * but then fill all MSR's with zeroes.
+	 * Some CPUs have turbo boost but don't declare any turbo ratio
+	 * in MSR_TURBO_RATIO_LIMIT.
 	 */
-	if (!base_freq) {
-		pr_debug("Couldn't determine cpu base frequency, necessary for scale-invariant accounting.\n");
+	if (!base_freq || !turbo_freq) {
+		pr_debug("Couldn't determine cpu base or turbo frequency, necessary for scale-invariant accounting.\n");
 		return false;
 	}
 
-- 
2.16.4

Re: [PATCH 1/2] x86, sched: Prevent divisions by zero in frequency invariant accounting

[Rafael J. Wysocki]

On Tue, Apr 28, 2020 at 3:25 PM Giovanni Gherdovich <ggherdovich@suse.cz> wrote:
>
> The product mcnt * arch_max_freq_ratio could be zero if it overflows u64.
>
> For context, a large value for arch_max_freq_ratio would be 5000,
> corresponding to a turbo_freq/base_freq ratio of 5 (normally it's more like
> 1500-2000). A large increment frequency for the MPERF counter would be 5GHz
> (the base clock of all CPUs on the market today is less than that). With
> these figures, a CPU would need to go without a scheduler tick for around 8
> days for the u64 overflow to happen. It is unlikely, but the check is
> warranted.
>
> In that case it's also appropriate to disable frequency invariant
> accounting: the feature relies on measures of the clock frequency done at
> every scheduler tick, which need to be "fresh" to be at all meaningful.
>
> Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
> Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

> ---
>  arch/x86/kernel/smpboot.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 8c89e4d9ad28..4718f29a3065 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -2039,6 +2039,14 @@ static void init_freq_invariance(bool secondary)
>         }
>  }
>
> +static void disable_freq_invariance_workfn(struct work_struct *work)
> +{
> +       static_branch_disable(&arch_scale_freq_key);
> +}
> +
> +static DECLARE_WORK(disable_freq_invariance_work,
> +                   disable_freq_invariance_workfn);
> +
>  DEFINE_PER_CPU(unsigned long, arch_freq_scale) = SCHED_CAPACITY_SCALE;
>
>  void arch_scale_freq_tick(void)
> @@ -2055,14 +2063,18 @@ void arch_scale_freq_tick(void)
>
>         acnt = aperf - this_cpu_read(arch_prev_aperf);
>         mcnt = mperf - this_cpu_read(arch_prev_mperf);
> -       if (!mcnt)
> -               return;
>
>         this_cpu_write(arch_prev_aperf, aperf);
>         this_cpu_write(arch_prev_mperf, mperf);
>
>         acnt <<= 2*SCHED_CAPACITY_SHIFT;
>         mcnt *= arch_max_freq_ratio;
> +       if (!mcnt) {
> +               pr_warn("Scheduler tick missing for long time, disabling scale-invariant accounting.\n");
> +               /* static_branch_disable() acquires a lock and may sleep */
> +               schedule_work(&disable_freq_invariance_work);
> +               return;
> +       }
>
>         freq_scale = div64_u64(acnt, mcnt);
>
> --
> 2.16.4
>

Re: [PATCH 2/2] x86, sched: Bail out of frequency invariance if turbo frequency is unknown

[Rafael J. Wysocki]

On Tue, Apr 28, 2020 at 3:25 PM Giovanni Gherdovich <ggherdovich@suse.cz> wrote:
>
> There may be CPUs that support turbo boost but don't declare any turbo
> ratio, i.e. their MSR_TURBO_RATIO_LIMIT is all zeroes. In that condition
> scale-invariant calculations can't be performed.
>
> Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
> Suggested-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

> ---
>  arch/x86/kernel/smpboot.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 4718f29a3065..ab2a0df7d1fb 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1991,9 +1991,11 @@ static bool intel_set_max_freq_ratio(void)
>         /*
>          * Some hypervisors advertise X86_FEATURE_APERFMPERF
>          * but then fill all MSR's with zeroes.
> +        * Some CPUs have turbo boost but don't declare any turbo ratio
> +        * in MSR_TURBO_RATIO_LIMIT.
>          */
> -       if (!base_freq) {
> -               pr_debug("Couldn't determine cpu base frequency, necessary for scale-invariant accounting.\n");
> +       if (!base_freq || !turbo_freq) {
> +               pr_debug("Couldn't determine cpu base or turbo frequency, necessary for scale-invariant accounting.\n");
>                 return false;
>         }
>
> --
> 2.16.4
>

Re: [PATCH 2/2] x86, sched: Bail out of frequency invariance if turbo frequency is unknown

[Peter Zijlstra]

On Tue, Apr 28, 2020 at 03:24:50PM +0200, Giovanni Gherdovich wrote:
> There may be CPUs that support turbo boost but don't declare any turbo
> ratio, i.e. their MSR_TURBO_RATIO_LIMIT is all zeroes. In that condition
> scale-invariant calculations can't be performed.
> 
> Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
> Suggested-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")
> ---
>  arch/x86/kernel/smpboot.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 4718f29a3065..ab2a0df7d1fb 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1991,9 +1991,11 @@ static bool intel_set_max_freq_ratio(void)
>  	/*
>  	 * Some hypervisors advertise X86_FEATURE_APERFMPERF
>  	 * but then fill all MSR's with zeroes.
> +	 * Some CPUs have turbo boost but don't declare any turbo ratio
> +	 * in MSR_TURBO_RATIO_LIMIT.
>  	 */
> -	if (!base_freq) {
> -		pr_debug("Couldn't determine cpu base frequency, necessary for scale-invariant accounting.\n");
> +	if (!base_freq || !turbo_freq) {
> +		pr_debug("Couldn't determine cpu base or turbo frequency, necessary for scale-invariant accounting.\n");
>  		return false;
>  	}

I've added the below, imagine base_freq > turbo_freq *
SCHED_CAPACITY_SCALE.

--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1975,6 +1975,7 @@ static bool core_set_max_freq_ratio(u64
 static bool intel_set_max_freq_ratio(void)
 {
 	u64 base_freq, turbo_freq;
+	u64 turbo_ratio;
 
 	if (slv_set_max_freq_ratio(&base_freq, &turbo_freq))
 		goto out;
@@ -2008,9 +2009,15 @@ static bool intel_set_max_freq_ratio(voi
 		return false;
 	}
 
-	arch_turbo_freq_ratio = div_u64(turbo_freq * SCHED_CAPACITY_SCALE,
-					base_freq);
+	turbo_ratio = div_u64(turbo_freq * SCHED_CAPACITY_SCALE, base_freq);
+	if (!turbo_ratio) {
+		pr_debug("Non-zero turbo and base frequencies led to a 0 ratio.\n");
+		return false;
+	}
+
+	arch_turbo_freq_ratio = turbo_ratio;
 	arch_set_max_freq_ratio(turbo_disabled());
+
 	return true;
 }
 

Re: [PATCH 1/2] x86, sched: Prevent divisions by zero in frequency invariant accounting

[Peter Zijlstra]

On Tue, Apr 28, 2020 at 03:24:49PM +0200, Giovanni Gherdovich wrote:
> The product mcnt * arch_max_freq_ratio could be zero if it overflows u64.
> 
> For context, a large value for arch_max_freq_ratio would be 5000,
> corresponding to a turbo_freq/base_freq ratio of 5 (normally it's more like
> 1500-2000). A large increment frequency for the MPERF counter would be 5GHz
> (the base clock of all CPUs on the market today is less than that). With
> these figures, a CPU would need to go without a scheduler tick for around 8
> days for the u64 overflow to happen. It is unlikely, but the check is
> warranted.
> 
> In that case it's also appropriate to disable frequency invariant
> accounting: the feature relies on measures of the clock frequency done at
> every scheduler tick, which need to be "fresh" to be at all meaningful.
> 
> Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
> Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")

>  	acnt <<= 2*SCHED_CAPACITY_SHIFT;
>  	mcnt *= arch_max_freq_ratio;
> +	if (!mcnt) {

The problem is; this doesn't do what you claim it does.

> +		pr_warn("Scheduler tick missing for long time, disabling scale-invariant accounting.\n");
> +		/* static_branch_disable() acquires a lock and may sleep */
> +		schedule_work(&disable_freq_invariance_work);
> +		return;
> +	}
>  
>  	freq_scale = div64_u64(acnt, mcnt);

I've changed the patch like so.. OK?

(ok, perhaps I went a little overboard with the paranoia ;-)

--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -55,6 +55,7 @@
 #include <linux/gfp.h>
 #include <linux/cpuidle.h>
 #include <linux/numa.h>
+#include <linux/overflow.h>
 
 #include <asm/acpi.h>
 #include <asm/desc.h>
@@ -2057,11 +2058,19 @@ static void init_freq_invariance(bool se
 	}
 }
 
+static void disable_freq_invariance_workfn(struct work_struct *work)
+{
+	static_branch_disable(&arch_scale_freq_key);
+}
+
+static DECLARE_WORK(disable_freq_invariance_work,
+		    disable_freq_invariance_workfn);
+
 DEFINE_PER_CPU(unsigned long, arch_freq_scale) = SCHED_CAPACITY_SCALE;
 
 void arch_scale_freq_tick(void)
 {
-	u64 freq_scale;
+	u64 freq_scale = SCHED_CAPACITY_SCALE;
 	u64 aperf, mperf;
 	u64 acnt, mcnt;
 
@@ -2073,19 +2082,27 @@ void arch_scale_freq_tick(void)
 
 	acnt = aperf - this_cpu_read(arch_prev_aperf);
 	mcnt = mperf - this_cpu_read(arch_prev_mperf);
-	if (!mcnt)
-		return;
 
 	this_cpu_write(arch_prev_aperf, aperf);
 	this_cpu_write(arch_prev_mperf, mperf);
 
-	acnt <<= 2*SCHED_CAPACITY_SHIFT;
-	mcnt *= arch_max_freq_ratio;
+	if (check_shl_overflow(acnt, 2*SCHED_CAPACITY_SHIFT, &acnt))
+		goto error;
+
+	if (check_mul_overflow(mcnt, arch_max_freq_ratio, &mcnt) || !mcnt)
+		goto error;
 
 	freq_scale = div64_u64(acnt, mcnt);
+	if (!freq_scale)
+		goto error;
 
 	if (freq_scale > SCHED_CAPACITY_SCALE)
 		freq_scale = SCHED_CAPACITY_SCALE;
 
 	this_cpu_write(arch_freq_scale, freq_scale);
+	return;
+
+error:
+	pr_warn("Scheduler frequency invariance went wobbly, disabling!\n");
+	schedule_work(&disable_freq_invariance_work);
 }

Re: [PATCH 2/2] x86, sched: Bail out of frequency invariance if turbo frequency is unknown

[Ricardo Neri]

On Tue, Apr 28, 2020 at 03:24:50PM +0200, Giovanni Gherdovich wrote:
> There may be CPUs that support turbo boost but don't declare any turbo
> ratio, i.e. their MSR_TURBO_RATIO_LIMIT is all zeroes. In that condition
> scale-invariant calculations can't be performed.
> 
> Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
> Suggested-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>

Thanks for implementing this, Giovanni!

Tested-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>

Re: [PATCH 2/2] x86, sched: Bail out of frequency invariance if turbo frequency is unknown

[Ricardo Neri]

On Fri, May 01, 2020 at 03:04:27PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 28, 2020 at 03:24:50PM +0200, Giovanni Gherdovich wrote:
> > There may be CPUs that support turbo boost but don't declare any turbo
> > ratio, i.e. their MSR_TURBO_RATIO_LIMIT is all zeroes. In that condition
> > scale-invariant calculations can't be performed.
> > 
> > Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
> > Suggested-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> > Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")
> > ---
> >  arch/x86/kernel/smpboot.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> > index 4718f29a3065..ab2a0df7d1fb 100644
> > --- a/arch/x86/kernel/smpboot.c
> > +++ b/arch/x86/kernel/smpboot.c
> > @@ -1991,9 +1991,11 @@ static bool intel_set_max_freq_ratio(void)
> >  	/*
> >  	 * Some hypervisors advertise X86_FEATURE_APERFMPERF
> >  	 * but then fill all MSR's with zeroes.
> > +	 * Some CPUs have turbo boost but don't declare any turbo ratio
> > +	 * in MSR_TURBO_RATIO_LIMIT.
> >  	 */
> > -	if (!base_freq) {
> > -		pr_debug("Couldn't determine cpu base frequency, necessary for scale-invariant accounting.\n");
> > +	if (!base_freq || !turbo_freq) {
> > +		pr_debug("Couldn't determine cpu base or turbo frequency, necessary for scale-invariant accounting.\n");
> >  		return false;
> >  	}
> 
> I've added the below, imagine base_freq > turbo_freq *
> SCHED_CAPACITY_SCALE.
> 
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1975,6 +1975,7 @@ static bool core_set_max_freq_ratio(u64
>  static bool intel_set_max_freq_ratio(void)
>  {
>  	u64 base_freq, turbo_freq;
> +	u64 turbo_ratio;
>  
>  	if (slv_set_max_freq_ratio(&base_freq, &turbo_freq))
>  		goto out;
> @@ -2008,9 +2009,15 @@ static bool intel_set_max_freq_ratio(voi
>  		return false;
>  	}
>  
> -	arch_turbo_freq_ratio = div_u64(turbo_freq * SCHED_CAPACITY_SCALE,
> -					base_freq);
> +	turbo_ratio = div_u64(turbo_freq * SCHED_CAPACITY_SCALE, base_freq);
> +	if (!turbo_ratio) {
> +		pr_debug("Non-zero turbo and base frequencies led to a 0 ratio.\n");
> +		return false;
> +	}
> +
> +	arch_turbo_freq_ratio = turbo_ratio;

I guess this covers more cases in which turbo_ratio can be zero.

Also, FWIW

Tested-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>

Thanks and BR,
Ricardo

Re: [PATCH 1/2] x86, sched: Prevent divisions by zero in frequency invariant accounting

[Giovanni Gherdovich]

On Fri, 2020-05-01 at 15:30 +0200, Peter Zijlstra wrote:
> On Tue, Apr 28, 2020 at 03:24:49PM +0200, Giovanni Gherdovich wrote:
> > The product mcnt * arch_max_freq_ratio could be zero if it overflows u64.
> > 
> > For context, a large value for arch_max_freq_ratio would be 5000,
> > corresponding to a turbo_freq/base_freq ratio of 5 (normally it's more like
> > 1500-2000). A large increment frequency for the MPERF counter would be 5GHz
> > (the base clock of all CPUs on the market today is less than that). With
> > these figures, a CPU would need to go without a scheduler tick for around 8
> > days for the u64 overflow to happen. It is unlikely, but the check is
> > warranted.
> > 
> > In that case it's also appropriate to disable frequency invariant
> > accounting: the feature relies on measures of the clock frequency done at
> > every scheduler tick, which need to be "fresh" to be at all meaningful.
> > 
> > Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
> > Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")
> >  	acnt <<= 2*SCHED_CAPACITY_SHIFT;
> >  	mcnt *= arch_max_freq_ratio;
> > +	if (!mcnt) {
> 
> The problem is; this doesn't do what you claim it does.
> 
> > +		pr_warn("Scheduler tick missing for long time, disabling scale-invariant accounting.\n");
> > +		/* static_branch_disable() acquires a lock and may sleep */
> > +		schedule_work(&disable_freq_invariance_work);
> > +		return;
> > +	}
> >  
> >  	freq_scale = div64_u64(acnt, mcnt);
> 
> I've changed the patch like so.. OK?
> 
> (ok, perhaps I went a little overboard with the paranoia ;-)

Right, I wasn't really checking for overflow, only for when the product
"mcnt * arch_max_freq_ratio" becomes zero.

Thanks for your edit (I took note of the macros check_*_overflow, didn't know
them). I fully subscribe to the paranoid approach.

I understand you've already edited the patches in your tree, so I am not
resending, just confirming my

Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>

> 
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -55,6 +55,7 @@
>  #include <linux/gfp.h>
>  #include <linux/cpuidle.h>
>  #include <linux/numa.h>
> +#include <linux/overflow.h>
>  
>  #include <asm/acpi.h>
>  #include <asm/desc.h>
> @@ -2057,11 +2058,19 @@ static void init_freq_invariance(bool se
>  	}
>  }
>  
> +static void disable_freq_invariance_workfn(struct work_struct *work)
> +{
> +	static_branch_disable(&arch_scale_freq_key);
> +}
> +
> +static DECLARE_WORK(disable_freq_invariance_work,
> +		    disable_freq_invariance_workfn);
> +
>  DEFINE_PER_CPU(unsigned long, arch_freq_scale) = SCHED_CAPACITY_SCALE;
>  
>  void arch_scale_freq_tick(void)
>  {
> -	u64 freq_scale;
> +	u64 freq_scale = SCHED_CAPACITY_SCALE;
>  	u64 aperf, mperf;
>  	u64 acnt, mcnt;
>  
> @@ -2073,19 +2082,27 @@ void arch_scale_freq_tick(void)
>  
>  	acnt = aperf - this_cpu_read(arch_prev_aperf);
>  	mcnt = mperf - this_cpu_read(arch_prev_mperf);
> -	if (!mcnt)
> -		return;
>  
>  	this_cpu_write(arch_prev_aperf, aperf);
>  	this_cpu_write(arch_prev_mperf, mperf);
>  
> -	acnt <<= 2*SCHED_CAPACITY_SHIFT;
> -	mcnt *= arch_max_freq_ratio;
> +	if (check_shl_overflow(acnt, 2*SCHED_CAPACITY_SHIFT, &acnt))
> +		goto error;
> +
> +	if (check_mul_overflow(mcnt, arch_max_freq_ratio, &mcnt) || !mcnt)
> +		goto error;
>  
>  	freq_scale = div64_u64(acnt, mcnt);
> +	if (!freq_scale)
> +		goto error;
>  
>  	if (freq_scale > SCHED_CAPACITY_SCALE)
>  		freq_scale = SCHED_CAPACITY_SCALE;
>  
>  	this_cpu_write(arch_freq_scale, freq_scale);
> +	return;
> +
> +error:
> +	pr_warn("Scheduler frequency invariance went wobbly, disabling!\n");
> +	schedule_work(&disable_freq_invariance_work);
>  }

Re: [PATCH 2/2] x86, sched: Bail out of frequency invariance if turbo frequency is unknown

[Giovanni Gherdovich]

On Fri, 2020-05-01 at 15:04 +0200, Peter Zijlstra wrote:
> On Tue, Apr 28, 2020 at 03:24:50PM +0200, Giovanni Gherdovich wrote:
> > There may be CPUs that support turbo boost but don't declare any turbo
> > ratio, i.e. their MSR_TURBO_RATIO_LIMIT is all zeroes. In that condition
> > scale-invariant calculations can't be performed.
> > 
> > Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
> > Suggested-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> > Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")
> > ---
> >  arch/x86/kernel/smpboot.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> > index 4718f29a3065..ab2a0df7d1fb 100644
> > --- a/arch/x86/kernel/smpboot.c
> > +++ b/arch/x86/kernel/smpboot.c
> > @@ -1991,9 +1991,11 @@ static bool intel_set_max_freq_ratio(void)
> >  	/*
> >  	 * Some hypervisors advertise X86_FEATURE_APERFMPERF
> >  	 * but then fill all MSR's with zeroes.
> > +	 * Some CPUs have turbo boost but don't declare any turbo ratio
> > +	 * in MSR_TURBO_RATIO_LIMIT.
> >  	 */
> > -	if (!base_freq) {
> > -		pr_debug("Couldn't determine cpu base frequency, necessary for scale-invariant accounting.\n");
> > +	if (!base_freq || !turbo_freq) {
> > +		pr_debug("Couldn't determine cpu base or turbo frequency, necessary for scale-invariant accounting.\n");
> >  		return false;
> >  	}
> 
> I've added the below, imagine base_freq > turbo_freq *
> SCHED_CAPACITY_SCALE.

Right, I didn't consider that case. It doesn't hurt to be defensive.

I understand you've already edited the patches in your tree, so I am not
resending, just confirming my

Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>

> 
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1975,6 +1975,7 @@ static bool core_set_max_freq_ratio(u64
>  static bool intel_set_max_freq_ratio(void)
>  {
>  	u64 base_freq, turbo_freq;
> +	u64 turbo_ratio;
>  
>  	if (slv_set_max_freq_ratio(&base_freq, &turbo_freq))
>  		goto out;
> @@ -2008,9 +2009,15 @@ static bool intel_set_max_freq_ratio(voi
>  		return false;
>  	}
>  
> -	arch_turbo_freq_ratio = div_u64(turbo_freq * SCHED_CAPACITY_SCALE,
> -					base_freq);
> +	turbo_ratio = div_u64(turbo_freq * SCHED_CAPACITY_SCALE, base_freq);
> +	if (!turbo_ratio) {
> +		pr_debug("Non-zero turbo and base frequencies led to a 0 ratio.\n");
> +		return false;
> +	}
> +
> +	arch_turbo_freq_ratio = turbo_ratio;
>  	arch_set_max_freq_ratio(turbo_disabled());
> +
>  	return true;
>  }
>  

Re: [PATCH 1/2] x86, sched: Prevent divisions by zero in frequency invariant accounting

[Ricardo Neri]

On Sat, May 02, 2020 at 04:25:00PM +0200, Giovanni Gherdovich wrote:
> > 
> > I've changed the patch like so.. OK?
> > 
> > (ok, perhaps I went a little overboard with the paranoia ;-)
> 
> Right, I wasn't really checking for overflow, only for when the product
> "mcnt * arch_max_freq_ratio" becomes zero.
> 
> Thanks for your edit (I took note of the macros check_*_overflow, didn't know
> them). I fully subscribe to the paranoid approach.
> 
> I understand you've already edited the patches in your tree, so I am not
> resending, just confirming my
> 
> Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>

Hi, have these changes been merged? I still don't see them in the tip or
Linus' tree.

Thanks and BR,
Ricardo

Re: [PATCH 1/2] x86, sched: Prevent divisions by zero in frequency invariant accounting

[Giovanni Gherdovich]

On Mon, 2020-05-18 at 15:20 -0700, Ricardo Neri wrote:
> On Sat, May 02, 2020 at 04:25:00PM +0200, Giovanni Gherdovich wrote:
> > > 
> > > I've changed the patch like so.. OK?
> > > 
> > > (ok, perhaps I went a little overboard with the paranoia ;-)
> > 
> > Right, I wasn't really checking for overflow, only for when the product
> > "mcnt * arch_max_freq_ratio" becomes zero.
> > 
> > Thanks for your edit (I took note of the macros check_*_overflow, didn't know
> > them). I fully subscribe to the paranoid approach.
> > 
> > I understand you've already edited the patches in your tree, so I am not
> > resending, just confirming my
> > 
> > Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
> 
> Hi, have these changes been merged? I still don't see them in the tip or
> Linus' tree.
> 

Hi Ricardo,

the kbuild bot found an error in this patch, the macro check_mul_overflow
doesn't build on x86 32bit, so Peter Zijlstra hasn't merged it yet.
This is the error:
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org/thread/7GDIBOMNVDG5W2XZD4EICE2TUZR3THBN/

I'm writing a patch to avoid doing frequency invariance entirely on i386.
I doubt those machines have APERFMPERF anyways. This will fix the build error.


Cheers,
Giovanni