diff mbox series

[v3] PM: EM: Fix potential division-by-zero error in em_compute_costs()

Message ID tencent_6D2374392DB66C9D23BF6E2546638A42EC08@qq.com
State New
Headers show
Series [v3] PM: EM: Fix potential division-by-zero error in em_compute_costs() | expand

Commit Message

Yaxiong Tian April 14, 2025, 9:04 a.m. UTC 
From: Yaxiong Tian <tianyaxiong@kylinos.cn>

When the device is of a non-CPU type, table[i].performance won't be
initialized in the previous em_init_performance(), resulting in division
by zero when calculating costs in em_compute_costs().

Since the 'cost' algorithm is only used for EAS energy efficiency
calculations and is currently not utilized by other device drivers, we
should add the _is_cpu_device(dev) check to prevent this division-by-zero
issue.

Fixes: <1b600da51073> ("PM: EM: Optimize em_cpu_energy() and remove division")
Signed-off-by: Yaxiong Tian <tianyaxiong@kylinos.cn>
---
 kernel/power/energy_model.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Lukasz Luba April 15, 2025, 8:50 a.m. UTC | #1 
On 4/14/25 10:04, Yaxiong Tian wrote:
> From: Yaxiong Tian <tianyaxiong@kylinos.cn>
> 
> When the device is of a non-CPU type, table[i].performance won't be
> initialized in the previous em_init_performance(), resulting in division
> by zero when calculating costs in em_compute_costs().
> 
> Since the 'cost' algorithm is only used for EAS energy efficiency
> calculations and is currently not utilized by other device drivers, we
> should add the _is_cpu_device(dev) check to prevent this division-by-zero
> issue.
> 
> Fixes: <1b600da51073> ("PM: EM: Optimize em_cpu_energy() and remove division")
> Signed-off-by: Yaxiong Tian <tianyaxiong@kylinos.cn>
> ---
>   kernel/power/energy_model.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
> index d9b7e2b38c7a..fc972cc1fc12 100644
> --- a/kernel/power/energy_model.c
> +++ b/kernel/power/energy_model.c
> @@ -235,7 +235,7 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
>   
>   	/* Compute the cost of each performance state. */
>   	for (i = nr_states - 1; i >= 0; i--) {
> -		unsigned long power_res, cost;
> +		unsigned long power_res, cost = 0;
>   
>   		if ((flags & EM_PERF_DOMAIN_ARTIFICIAL) && cb->get_cost) {
>   			ret = cb->get_cost(dev, table[i].frequency, &cost);
> @@ -244,7 +244,7 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
>   					cost, ret);
>   				return -EINVAL;
>   			}
> -		} else {
> +		} else if (_is_cpu_device(dev)) {
>   			/* increase resolution of 'cost' precision */
>   			power_res = table[i].power * 10;
>   			cost = power_res / table[i].performance;

LGTM,

Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Rafael J. Wysocki April 15, 2025, 5:17 p.m. UTC | #2 
On Mon, Apr 14, 2025 at 11:09 AM Yaxiong Tian <iambestgod@qq.com> wrote:
>
> From: Yaxiong Tian <tianyaxiong@kylinos.cn>
>
> When the device is of a non-CPU type, table[i].performance won't be
> initialized in the previous em_init_performance(), resulting in division
> by zero when calculating costs in em_compute_costs().
>
> Since the 'cost' algorithm is only used for EAS energy efficiency
> calculations and is currently not utilized by other device drivers, we
> should add the _is_cpu_device(dev) check to prevent this division-by-zero
> issue.
>
> Fixes: <1b600da51073> ("PM: EM: Optimize em_cpu_energy() and remove division")

Please look at the Fixes: tags in the kernel git history.  They don't
look like the one above.

> Signed-off-by: Yaxiong Tian <tianyaxiong@kylinos.cn>
> ---
>  kernel/power/energy_model.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
> index d9b7e2b38c7a..fc972cc1fc12 100644
> --- a/kernel/power/energy_model.c
> +++ b/kernel/power/energy_model.c
> @@ -235,7 +235,7 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
>
>         /* Compute the cost of each performance state. */
>         for (i = nr_states - 1; i >= 0; i--) {
> -               unsigned long power_res, cost;
> +               unsigned long power_res, cost = 0;
>
>                 if ((flags & EM_PERF_DOMAIN_ARTIFICIAL) && cb->get_cost) {
>                         ret = cb->get_cost(dev, table[i].frequency, &cost);
> @@ -244,7 +244,7 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
>                                         cost, ret);
>                                 return -EINVAL;
>                         }
> -               } else {
> +               } else if (_is_cpu_device(dev)) {

Can't you just check this upfront at the beginning of the function and
make it bail out if dev is not a CPU device?

>                         /* increase resolution of 'cost' precision */
>                         power_res = table[i].power * 10;
>                         cost = power_res / table[i].performance;
> --
Yaxiong Tian April 16, 2025, 2:56 a.m. UTC | #3 
在 2025/4/16 01:17, Rafael J. Wysocki 写道:
> On Mon, Apr 14, 2025 at 11:09 AM Yaxiong Tian <iambestgod@qq.com> wrote:
>>
>> From: Yaxiong Tian <tianyaxiong@kylinos.cn>
>>
>> When the device is of a non-CPU type, table[i].performance won't be
>> initialized in the previous em_init_performance(), resulting in division
>> by zero when calculating costs in em_compute_costs().
>>
>> Since the 'cost' algorithm is only used for EAS energy efficiency
>> calculations and is currently not utilized by other device drivers, we
>> should add the _is_cpu_device(dev) check to prevent this division-by-zero
>> issue.
>>
>> Fixes: <1b600da51073> ("PM: EM: Optimize em_cpu_energy() and remove division")
> 
> Please look at the Fixes: tags in the kernel git history.  They don't
> look like the one above.
> 
Yes, there's an extra '<>' here.

>> Signed-off-by: Yaxiong Tian <tianyaxiong@kylinos.cn>
>> ---
>>   kernel/power/energy_model.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
>> index d9b7e2b38c7a..fc972cc1fc12 100644
>> --- a/kernel/power/energy_model.c
>> +++ b/kernel/power/energy_model.c
>> @@ -235,7 +235,7 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
>>
>>          /* Compute the cost of each performance state. */
>>          for (i = nr_states - 1; i >= 0; i--) {
>> -               unsigned long power_res, cost;
>> +               unsigned long power_res, cost = 0;
>>
>>                  if ((flags & EM_PERF_DOMAIN_ARTIFICIAL) && cb->get_cost) {
>>                          ret = cb->get_cost(dev, table[i].frequency, &cost);
>> @@ -244,7 +244,7 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
>>                                          cost, ret);
>>                                  return -EINVAL;
>>                          }
>> -               } else {
>> +               } else if (_is_cpu_device(dev)) {
> 
> Can't you just check this upfront at the beginning of the function and
> make it bail out if dev is not a CPU device?
> 
Sure, But the current implementation applies em_compute_costs() to both 
non-CPU devices and CPU devices. After carefully reviewing the latest code,
I've found this issue has expanded in scope.

There are currently three call paths for invoking em_compute_costs():

1) Registering performance domains (for both non-CPU and CPU devices)
em_dev_register_perf_domain() → em_create_pd() →
em_create_perf_table() → em_compute_costs()

2)EM update paths (CPU devices only)

Periodic 1000ms update check via em_update_work work item:
em_check_capacity_update() → em_adjust_new_capacity() → 
em_recalc_and_update() → em_compute_costs()

Exynos-chip initialization:
em_dev_update_chip_binning() → em_recalc_and_update() → em_compute_costs()

3) Device cost computation (non-CPU devices only - currently unused)
em_dev_compute_costs() → em_compute_costs()

Note: In em_dev_compute_costs(), when calling em_compute_costs(),
neither the callback (cb) nor flags are set.In fact, it either does
nothing at all or performs incorrect operations.

Therefore, should we mandate that non-CPU devices must provide a
get_cost callback?

So Should we add a check at the beginning of the em_compute_costs() to:

	if (!_is_cpu_device(dev) && !cb->get_cost) {
		dev_dbg(dev, "EM: No get_cost provided, cost unset.\n");
		return 0;
	}
And Modify em_dev_compute_costs() to require callers to provide the cb
callback function,Also need to update its corresponding comments.


>>                          /* increase resolution of 'cost' precision */
>>                          power_res = table[i].power * 10;
>>                          cost = power_res / table[i].performance;
>> --
Rafael J. Wysocki April 16, 2025, 11:58 a.m. UTC | #4 
On Wed, Apr 16, 2025 at 4:57 AM Yaxiong Tian <iambestgod@qq.com> wrote:
>
> 在 2025/4/16 01:17, Rafael J. Wysocki 写道:
> > On Mon, Apr 14, 2025 at 11:09 AM Yaxiong Tian <iambestgod@qq.com> wrote:
> >>
> >> From: Yaxiong Tian <tianyaxiong@kylinos.cn>
> >>
> >> When the device is of a non-CPU type, table[i].performance won't be
> >> initialized in the previous em_init_performance(), resulting in division
> >> by zero when calculating costs in em_compute_costs().
> >>
> >> Since the 'cost' algorithm is only used for EAS energy efficiency
> >> calculations and is currently not utilized by other device drivers, we
> >> should add the _is_cpu_device(dev) check to prevent this division-by-zero
> >> issue.
> >>
> >> Fixes: <1b600da51073> ("PM: EM: Optimize em_cpu_energy() and remove division")
> >
> > Please look at the Fixes: tags in the kernel git history.  They don't
> > look like the one above.
> >
> Yes, there's an extra '<>' here.
>
> >> Signed-off-by: Yaxiong Tian <tianyaxiong@kylinos.cn>
> >> ---
> >>   kernel/power/energy_model.c | 4 ++--
> >>   1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
> >> index d9b7e2b38c7a..fc972cc1fc12 100644
> >> --- a/kernel/power/energy_model.c
> >> +++ b/kernel/power/energy_model.c
> >> @@ -235,7 +235,7 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
> >>
> >>          /* Compute the cost of each performance state. */
> >>          for (i = nr_states - 1; i >= 0; i--) {
> >> -               unsigned long power_res, cost;
> >> +               unsigned long power_res, cost = 0;
> >>
> >>                  if ((flags & EM_PERF_DOMAIN_ARTIFICIAL) && cb->get_cost) {
> >>                          ret = cb->get_cost(dev, table[i].frequency, &cost);
> >> @@ -244,7 +244,7 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
> >>                                          cost, ret);
> >>                                  return -EINVAL;
> >>                          }
> >> -               } else {
> >> +               } else if (_is_cpu_device(dev)) {
> >
> > Can't you just check this upfront at the beginning of the function and
> > make it bail out if dev is not a CPU device?
> >
> Sure, But the current implementation applies em_compute_costs() to both
> non-CPU devices and CPU devices.

Maybe it shouldn't do that for non-CPU ones?

> After carefully reviewing the latest code,
> I've found this issue has expanded in scope.
>
> There are currently three call paths for invoking em_compute_costs():
>
> 1) Registering performance domains (for both non-CPU and CPU devices)
> em_dev_register_perf_domain() → em_create_pd() →
> em_create_perf_table() → em_compute_costs()
>
> 2)EM update paths (CPU devices only)
>
> Periodic 1000ms update check via em_update_work work item:
> em_check_capacity_update() → em_adjust_new_capacity() →
> em_recalc_and_update() → em_compute_costs()
>
> Exynos-chip initialization:
> em_dev_update_chip_binning() → em_recalc_and_update() → em_compute_costs()
>
> 3) Device cost computation (non-CPU devices only - currently unused)
> em_dev_compute_costs() → em_compute_costs()

So because this one is unused and AFAICS the cost values are never
used for non-CPU devices, it's better to just avoid computing them at
all.

> Note: In em_dev_compute_costs(), when calling em_compute_costs(),
> neither the callback (cb) nor flags are set.In fact, it either does
> nothing at all or performs incorrect operations.
>
> Therefore, should we mandate that non-CPU devices must provide a
> get_cost callback?

Why would that be an improvement?

> So Should we add a check at the beginning of the em_compute_costs() to:
>
>         if (!_is_cpu_device(dev) && !cb->get_cost) {
>                 dev_dbg(dev, "EM: No get_cost provided, cost unset.\n");
>                 return 0;
>         }
> And Modify em_dev_compute_costs() to require callers to provide the cb
> callback function,Also need to update its corresponding comments.
>
>
> >>                          /* increase resolution of 'cost' precision */
> >>                          power_res = table[i].power * 10;
> >>                          cost = power_res / table[i].performance;
> >> --

I think until there is a user of em_dev_compute_costs() this is all
moot and hard to figure out.

I would drop em_dev_compute_costs() altogether for now and put a
_is_cpu_device(dev) upfront check into em_compute_costs().
Lukasz Luba April 16, 2025, 3:26 p.m. UTC | #5 
On 4/16/25 12:58, Rafael J. Wysocki wrote:
> On Wed, Apr 16, 2025 at 4:57 AM Yaxiong Tian <iambestgod@qq.com> wrote:
>>
>> 在 2025/4/16 01:17, Rafael J. Wysocki 写道:
>>> On Mon, Apr 14, 2025 at 11:09 AM Yaxiong Tian <iambestgod@qq.com> wrote:
>>>>
>>>> From: Yaxiong Tian <tianyaxiong@kylinos.cn>
>>>>
>>>> When the device is of a non-CPU type, table[i].performance won't be
>>>> initialized in the previous em_init_performance(), resulting in division
>>>> by zero when calculating costs in em_compute_costs().
>>>>
>>>> Since the 'cost' algorithm is only used for EAS energy efficiency
>>>> calculations and is currently not utilized by other device drivers, we
>>>> should add the _is_cpu_device(dev) check to prevent this division-by-zero
>>>> issue.
>>>>
>>>> Fixes: <1b600da51073> ("PM: EM: Optimize em_cpu_energy() and remove division")
>>>
>>> Please look at the Fixes: tags in the kernel git history.  They don't
>>> look like the one above.
>>>
>> Yes, there's an extra '<>' here.
>>
>>>> Signed-off-by: Yaxiong Tian <tianyaxiong@kylinos.cn>
>>>> ---
>>>>    kernel/power/energy_model.c | 4 ++--
>>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
>>>> index d9b7e2b38c7a..fc972cc1fc12 100644
>>>> --- a/kernel/power/energy_model.c
>>>> +++ b/kernel/power/energy_model.c
>>>> @@ -235,7 +235,7 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
>>>>
>>>>           /* Compute the cost of each performance state. */
>>>>           for (i = nr_states - 1; i >= 0; i--) {
>>>> -               unsigned long power_res, cost;
>>>> +               unsigned long power_res, cost = 0;
>>>>
>>>>                   if ((flags & EM_PERF_DOMAIN_ARTIFICIAL) && cb->get_cost) {
>>>>                           ret = cb->get_cost(dev, table[i].frequency, &cost);
>>>> @@ -244,7 +244,7 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
>>>>                                           cost, ret);
>>>>                                   return -EINVAL;
>>>>                           }
>>>> -               } else {
>>>> +               } else if (_is_cpu_device(dev)) {
>>>
>>> Can't you just check this upfront at the beginning of the function and
>>> make it bail out if dev is not a CPU device?
>>>
>> Sure, But the current implementation applies em_compute_costs() to both
>> non-CPU devices and CPU devices.
> 
> Maybe it shouldn't do that for non-CPU ones?

It shouldn't call this cost computation for non-CPU devices.
Let me check that.
diff mbox series

Patch

diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
index d9b7e2b38c7a..fc972cc1fc12 100644
--- a/kernel/power/energy_model.c
+++ b/kernel/power/energy_model.c
@@ -235,7 +235,7 @@  static int em_compute_costs(struct device *dev, struct em_perf_state *table,
 
 	/* Compute the cost of each performance state. */
 	for (i = nr_states - 1; i >= 0; i--) {
-		unsigned long power_res, cost;
+		unsigned long power_res, cost = 0;
 
 		if ((flags & EM_PERF_DOMAIN_ARTIFICIAL) && cb->get_cost) {
 			ret = cb->get_cost(dev, table[i].frequency, &cost);
@@ -244,7 +244,7 @@  static int em_compute_costs(struct device *dev, struct em_perf_state *table,
 					cost, ret);
 				return -EINVAL;
 			}
-		} else {
+		} else if (_is_cpu_device(dev)) {
 			/* increase resolution of 'cost' precision */
 			power_res = table[i].power * 10;
 			cost = power_res / table[i].performance;