Re: [PATCH v2] powerpc/perf: Use cpumask_last() to determine the designated cpu for nest/core units.
From: Leonardo Bras <hidden>
Date: 2019-06-10 18:49:59
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?
quoted hunk ↗ jump to hunk
Signed-off-by: Anju T Sudhakar <redacted> Reviewed-by: Madhavan Srinivasan <redacted> --- 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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