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 <a...@linux.vnet.ibm.com>
> Reviewed-by: Madhavan Srinivasan <ma...@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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