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

From: Michael Ellerman <[email-masked]>
Date: Sun, 30 Jun 2019 18:37:25 +1000 (AEST)

On Mon, 2019-06-10 at 06:32:29 UTC, 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.
> 
> Example:
> In a system which has 2 sockets, with
> NUMA node0 CPU(s):     0-87
> NUMA node8 CPU(s):     88-175
> 
> Time taken to offline cpu 88-175:
> real    2m56.099s
> user    0m0.191s
> sys     0m0.000s
> 
> Use cpumask_last() to choose the target cpu, when the designated cpu
> goes online, so the migration will happen only when the last_cpu in the
> mask goes offline. This way the time taken to offline all cpus in a
> chip/core can be reduced.
> 
> With the patch, 
> 
> Time taken  to offline cpu 88-175:
> real    0m12.207s
> user    0m0.171s
> sys     0m0.000s
> 
> 
> Offlining all cpus in reverse order is also taken care because,
> cpumask_any_but() is used to find the designated cpu if the last cpu in
> the mask goes offline. Since cpumask_any_but() always return the first
> cpu in the mask, that becomes the designated cpu and migration will happen
> only when the first_cpu in the mask goes offline.
> 
> Example:
> With the patch,
> 
> Time taken to offline cpu from 175-88:
> real    0m9.330s
> user    0m0.110s
> sys     0m0.000s
> 
> Signed-off-by: Anju T Sudhakar <[email-masked]>
> Reviewed-by: Madhavan Srinivasan <[email-masked]>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/9c9f8fb71feed144973a70455e0a4ee3da57ed2a

cheers