Re: [PATCH] powerpc/perf: Use cpumask_last() to determine the

Hi,

Somehow the subject of this patch didn't appear completely here.

The Subject of this patch is as follows,

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

Thanks,

Anju


On 5/20/19 2:35 PM, 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.