[PATCH v2] powerpc/perf: Use cpumask_last() to determine the designated cpu for nest/core units.

[Anju T Sudhakar]

Hi Leonardo,

On 6/11/19 12:17 AM, Leonardo Bras wrote:
> On Mon, 2019-06-10 at 12:02 +0530, Anju T Sudhakar wrote:
>> Nest and core imc(In-memory Collection counters) assigns a particular
>> cpu as the designated target for counter data collection.
>> During system boot, the first online cpu in a chip gets assigned as
>> the designated cpu for that chip(for nest-imc) and the first online cpu
>> in a core gets assigned as the designated cpu for that core(for core-imc).
>>
>> If the designated cpu goes offline, the next online cpu from the same
>> chip(for nest-imc)/core(for core-imc) is assigned as the next target,
>> and the event context is migrated to the target cpu.
>> Currently, cpumask_any_but() function is used to find the target cpu.
>> Though this function is expected to return a `random` cpu, this always
>> returns the next online cpu.
>>
>> If all cpus in a chip/core is offlined in a sequential manner, starting
>> from the first cpu, the event migration has to happen for all the cpus
>> which goes offline. Since the migration process involves a grace period,
>> the total time taken to offline all the cpus will be significantly high.
> Seems like a very interesting work.
> Out of curiosity, have you used 'chcpu -d' to create your benchmark?

Here I did not use chcpu to disable the cpu.

I used a script which will offline cpus 88-175 by echoing  `0` to

/sys/devices/system/cpu/cpu*/online.


Regards,

Anju