Commit e0a79f52 authored by Mike Galbraith's avatar Mike Galbraith Committed by Ingo Molnar
Browse files

sched: Fix select_idle_sibling() bouncing cow syndrome

If the previous CPU is cache affine and idle, select it.

The current implementation simply traverses the sd_llc domain,
taking the first idle CPU encountered, which walks buddy pairs
hand in hand over the package, inflicting excruciating pain.

1 tbench pair (worst case) in a 10 core + SMT package:

  pre   15.22 MB/sec 1 procs
  post 252.01 MB/sec 1 procs
Signed-off-by: default avatarMike Galbraith <>
Cc: Peter Zijlstra <>

Signed-off-by: default avatarIngo Molnar <>
parent 60334caf
......@@ -3252,25 +3252,18 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
static int select_idle_sibling(struct task_struct *p, int target)
int cpu = smp_processor_id();
int prev_cpu = task_cpu(p);
struct sched_domain *sd;
struct sched_group *sg;
int i;
int i = task_cpu(p);
* If the task is going to be woken-up on this cpu and if it is
* already idle, then it is the right target.
if (target == cpu && idle_cpu(cpu))
return cpu;
if (idle_cpu(target))
return target;
* If the task is going to be woken-up on the cpu where it previously
* ran and if it is currently idle, then it the right target.
* If the prevous cpu is cache affine and idle, don't be stupid.
if (target == prev_cpu && idle_cpu(prev_cpu))
return prev_cpu;
if (i != target && cpus_share_cache(i, target) && idle_cpu(i))
return i;
* Otherwise, iterate the domains and find an elegible idle cpu.
......@@ -3284,7 +3277,7 @@ static int select_idle_sibling(struct task_struct *p, int target)
goto next;
for_each_cpu(i, sched_group_cpus(sg)) {
if (!idle_cpu(i))
if (i == target || !idle_cpu(i))
goto next;
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