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

[Madhavan Srinivasan]

On 20/05/19 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.
>
> 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
>
> cpumask_last() is a better way to find the target cpu, since in most of the
> cases cpuhotplug is performed in an increasing order(even in ppc64_cpu).
>
> cpumask_any_but() can still be used to check the possibility of other
> online cpus from the same chip/core if the last cpu in the mask goes
> offline.

Reviewed-by: Madhavan Srinivasan <maddy at linux.vnet.ibm.com>

Also add "Fixes:" tag and this should go to stable right?
>
> Signed-off-by: Anju T Sudhakar <anju at linux.vnet.ibm.com>
> ---
>   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);