Re: [PATCH v6 19/23] virt/steal_monitor: Provide default method to get num of CPUs for steal ratio

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 7/7/26 12:56 AM, Yury Norov wrote:
> On Wed, Jul 01, 2026 at 07:46:50PM +0530, Shrikanth Hegde wrote:
>> This method informs the steal_monitor core, how many CPUs it needs to
>> consider for steal ratio calculations.
>> steal_ratio = (delta_steal * 100 * 100) / (delta_ns * number_of_cpus);
>>
>> Default method returns number of Active CPUs since it calculates steal
>> time across active CPUs.
>>
>> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
>> ---
>> v5->v6:
>> - Add cpus_read_lock() for hotplug safety.
>>
>>   drivers/virt/steal_monitor/defaults.c | 11 +++++++++++
>>   drivers/virt/steal_monitor/sm_core.h  |  1 +
>>   2 files changed, 12 insertions(+)
>>
>> diff --git a/drivers/virt/steal_monitor/defaults.c b/drivers/virt/steal_monitor/defaults.c
>> index 4e2e5b233948..70dcfb1ce4cb 100644
>> --- a/drivers/virt/steal_monitor/defaults.c
>> +++ b/drivers/virt/steal_monitor/defaults.c
>> @@ -27,6 +27,17 @@ u64 __weak get_system_steal_time(void)
>>   	return total_steal;
>>   }
>>   
>> +/*
>> + * Return number of CPUs to consider for steal ratio calculation
>> + *
>> + * Default returns number of active CPUs.
>> + */
>> +unsigned int __weak get_num_cpus_steal_ratio(void)
>> +{
>> +	guard(cpus_read_lock)();
>> +	return num_active_cpus();
>> +}
> 
> How is this lock supposed to work? When you exit the scope, you can be
> interrupted and your caller will get outdated information.
> 

> The other callers of num_active_cpus() don't bother taking the lock.
> 

I am thinking i will switch to possible CPUs for steal ratio calculations.

That way cumulative steal_values are monotonically increasing and safe against
hotplug. That leaves the necessary for locks too.

For the SUT it is okay since possible==active. In any systems where it is not,
they have edit their default values a bit. Nothing else changes.


>> +
>>   /*
>>    * Default implementation of decrementing the preferred CPUs based on steal
>>    * time. This is simple logic and decrease the preferred CPUs by 1 core.
>> diff --git a/drivers/virt/steal_monitor/sm_core.h b/drivers/virt/steal_monitor/sm_core.h
>> index 030f6236c38e..794d3be04248 100644
>> --- a/drivers/virt/steal_monitor/sm_core.h
>> +++ b/drivers/virt/steal_monitor/sm_core.h
>> @@ -26,6 +26,7 @@ struct steal_monitor {
>>   extern struct steal_monitor sm_core_ctx;
>>   
>>   u64 get_system_steal_time(void);
>> +unsigned int get_num_cpus_steal_ratio(void);
>>   void increase_preferred_cpus(struct steal_monitor *ctx);
>>   void decrease_preferred_cpus(struct steal_monitor *ctx);
>>   
>> -- 
>> 2.47.3