sched/fair: Optimize select_idle_cpu

commit 60588bfa223ff675b95f866249f90616613fbe31 upstream.

select_idle_cpu() will scan the LLC domain for idle CPUs,
it's always expensive. so the next commit :

	1ad3aaf3fcd2 ("sched/core: Implement new approach to scale select_idle_cpu()")

introduces a way to limit how many CPUs we scan.

But it consume some CPUs out of 'nr' that are not allowed
for the task and thus waste our attempts. The function
always return nr_cpumask_bits, and we can't find a CPU
which our task is allowed to run.

Cpumask may be too big, similar to select_idle_core(), use
per_cpu_ptr 'select_idle_mask' to prevent stack overflow.

Fixes: 1ad3aaf3fcd2 ("sched/core: Implement new approach to scale select_idle_cpu()")
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20191213024530.28052-1-cj.chengjian@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ba749f579714cbe02f502b02402b8467aff41c50..b0ee5eedeccd192b41389d948ffc939d451a81e3 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5828,6 +5828,7 @@ static inline int select_idle_smt(struct task_struct *p, int target)
  */
 static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int target)
 {
+       struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
        struct sched_domain *this_sd;
        u64 avg_cost, avg_idle;
        u64 time, cost;
@@ -5859,11 +5860,11 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 
        time = cpu_clock(this);
 
-       for_each_cpu_wrap(cpu, sched_domain_span(sd), target) {
+       cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
+
+       for_each_cpu_wrap(cpu, cpus, target) {
                if (!--nr)
                        return si_cpu;
-               if (!cpumask_test_cpu(cpu, p->cpus_ptr))
-                       continue;
                if (available_idle_cpu(cpu))
                        break;
                if (si_cpu == -1 && sched_idle_cpu(cpu))