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

[Leonardo Bras]

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.
> 
> 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

Seems like a very interesting work.
Out of curiosity, have you used 'chcpu -d' to create your benchmark?

> 
> Signed-off-by: Anju T Sudhakar <anju at linux.vnet.ibm.com>
> Reviewed-by: Madhavan Srinivasan <maddy at linux.vnet.ibm.com>
> ---
> 
> Changes from v1:
> 	Modified the commit log with more info.
> ---
> 
>  arch/powerpc/perf/imc-pmu.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/perf/imc-pmu.c b/arch/powerpc/perf/imc-pmu.c
> index 31fa753..fbfd6e7 100644
> --- a/arch/powerpc/perf/imc-pmu.c
> +++ b/arch/powerpc/perf/imc-pmu.c
> @@ -366,7 +366,14 @@ static int ppc_nest_imc_cpu_offline(unsigned int cpu)
>  	 */
>  	nid = cpu_to_node(cpu);
>  	l_cpumask = cpumask_of_node(nid);
> -	target = cpumask_any_but(l_cpumask, cpu);
> +	target = cpumask_last(l_cpumask);
> +
> +	/*
> +	 * If this(target) is the last cpu in the cpumask for this chip,
> +	 * check for any possible online cpu in the chip.
> +	 */
> +	if (unlikely(target == cpu))
> +		target = cpumask_any_but(l_cpumask, cpu);
>  
>  	/*
>  	 * Update the cpumask with the target cpu and
> @@ -671,7 +678,10 @@ static int ppc_core_imc_cpu_offline(unsigned int cpu)
>  		return 0;
>  
>  	/* Find any online cpu in that core except the current "cpu" */
> -	ncpu = cpumask_any_but(cpu_sibling_mask(cpu), cpu);
> +	ncpu = cpumask_last(cpu_sibling_mask(cpu));
> +
> +	if (unlikely(ncpu == cpu))
> +		ncpu = cpumask_any_but(cpu_sibling_mask(cpu), cpu);
>  
>  	if (ncpu >= 0 && ncpu < nr_cpu_ids) {
>  		cpumask_set_cpu(ncpu, &core_imc_cpumask);
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