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sched_ext: Fixes for v6.16-rc1

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Two fixes in the built-in idle selection helpers.
 
 - Fix prev_cpu handling to guarantee that idle selection never returns a CPU
   that's not allowed.
 
 - Skip cross-node search when !NUMA which could lead to infinite looping due
   to a bug in NUMA iterator.
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Merge tag 'sched_ext-for-6.16-rc1-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext

Pull sched_ext fixes from Tejun Heo:
 "Two fixes in the built-in idle selection helpers:

   - Fix prev_cpu handling to guarantee that idle selection never
     returns a CPU that's not allowed

   - Skip cross-node search when !NUMA which could lead to infinite
     looping due to a bug in NUMA iterator"

* tag 'sched_ext-for-6.16-rc1-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext:
  sched_ext: idle: Skip cross-node search with !CONFIG_NUMA
  sched_ext: idle: Properly handle invalid prev_cpu during idle selection
This commit is contained in:
Linus Torvalds 2025-06-04 12:07:16 -07:00
commit 16b70698aa
1 changed files with 19 additions and 18 deletions
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37
kernel/sched/ext_idle.c
View file

@ -138,6 +138,7 @@ found:
goto retry;
}
#ifdef CONFIG_NUMA
/*
* Tracks nodes that have not yet been visited when searching for an idle
* CPU across all available nodes.
@ -186,6 +187,13 @@ static s32 pick_idle_cpu_from_online_nodes(const struct cpumask *cpus_allowed, i
return cpu;
}
#else
static inline s32
pick_idle_cpu_from_online_nodes(const struct cpumask *cpus_allowed, int node, u64 flags)
{
return -EBUSY;
}
#endif
/*
* Find an idle CPU in the system, starting from @node.
@ -447,10 +455,17 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
const struct cpumask *llc_cpus = NULL, *numa_cpus = NULL;
const struct cpumask *allowed = cpus_allowed ?: p->cpus_ptr;
int node = scx_cpu_node_if_enabled(prev_cpu);
bool is_prev_allowed;
s32 cpu;
preempt_disable();
/*
* Check whether @prev_cpu is still within the allowed set. If not,
* we can still try selecting a nearby CPU.
*/
is_prev_allowed = cpumask_test_cpu(prev_cpu, allowed);
/*
* Determine the subset of CPUs usable by @p within @cpus_allowed.
*/
@ -465,21 +480,6 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
cpu = -EBUSY;
goto out_enable;
}
/*
* If @prev_cpu is not in the allowed CPUs, skip topology
* optimizations and try to pick any idle CPU usable by the
* task.
*
* If %SCX_OPS_BUILTIN_IDLE_PER_NODE is enabled, prioritize
* the current node, as it may optimize some waker->wakee
* workloads.
*/
if (!cpumask_test_cpu(prev_cpu, allowed)) {
node = scx_cpu_node_if_enabled(smp_processor_id());
cpu = scx_pick_idle_cpu(allowed, node, flags);
goto out_enable;
}
}
/*
@ -525,7 +525,7 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
* then avoid a migration.
*/
cpu = smp_processor_id();
if (cpus_share_cache(cpu, prev_cpu) &&
if (is_prev_allowed && cpus_share_cache(cpu, prev_cpu) &&
scx_idle_test_and_clear_cpu(prev_cpu)) {
cpu = prev_cpu;
goto out_unlock;
@ -562,7 +562,8 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
/*
* Keep using @prev_cpu if it's part of a fully idle core.
*/
if (cpumask_test_cpu(prev_cpu, idle_cpumask(node)->smt) &&
if (is_prev_allowed &&
cpumask_test_cpu(prev_cpu, idle_cpumask(node)->smt) &&
scx_idle_test_and_clear_cpu(prev_cpu)) {
cpu = prev_cpu;
goto out_unlock;
@ -611,7 +612,7 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
/*
* Use @prev_cpu if it's idle.
*/
if (scx_idle_test_and_clear_cpu(prev_cpu)) {
if (is_prev_allowed && scx_idle_test_and_clear_cpu(prev_cpu)) {
cpu = prev_cpu;
goto out_unlock;
}