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Merge tag 'sched_ext-for-6.17-rc7-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext

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Pull sched_ext fix from jun Heo:
 "This contains a fix for sched_ext idle CPU selection that likely fixes
  a substantial performance regression.

  The scx_bpf_select_cpu_dfl/and() kfuncs were incorrectly detecting all
  tasks as migration-disabled when called outside ops.select_cpu(),
  causing them to always return -EBUSY instead of finding idle CPUs.

  The fix properly distinguishes between genuinely migration-disabled
  tasks vs. the current task whose migration is temporarily disabled by
  BPF execution"

* tag 'sched_ext-for-6.17-rc7-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext:
  sched_ext: idle: Handle migration-disabled tasks in BPF code
This commit is contained in:
Linus Torvalds
2025-09-22 11:28:52 -07:00
parent d4c7fccfa7 55ed11b181
commit cec1e6e5d1
1 changed files with 27 additions and 1 deletions
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28
kernel/sched/ext_idle.c
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@@ -856,6 +856,32 @@ static bool check_builtin_idle_enabled(void)
return false;
}
/*
* Determine whether @p is a migration-disabled task in the context of BPF
* code.
*
* We can't simply check whether @p->migration_disabled is set in a
* sched_ext callback, because migration is always disabled for the current
* task while running BPF code.
*
* The prolog (__bpf_prog_enter) and epilog (__bpf_prog_exit) respectively
* disable and re-enable migration. For this reason, the current task
* inside a sched_ext callback is always a migration-disabled task.
*
* Therefore, when @p->migration_disabled == 1, check whether @p is the
* current task or not: if it is, then migration was not disabled before
* entering the callback, otherwise migration was disabled.
*
* Returns true if @p is migration-disabled, false otherwise.
*/
static bool is_bpf_migration_disabled(const struct task_struct *p)
{
if (p->migration_disabled == 1)
return p != current;
else
return p->migration_disabled;
}
static s32 select_cpu_from_kfunc(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
const struct cpumask *allowed, u64 flags)
{
@@ -898,7 +924,7 @@ static s32 select_cpu_from_kfunc(struct task_struct *p, s32 prev_cpu, u64 wake_f
* selection optimizations and simply check whether the previously
* used CPU is idle and within the allowed cpumask.
*/
if (p->nr_cpus_allowed == 1 || is_migration_disabled(p)) {
if (p->nr_cpus_allowed == 1 || is_bpf_migration_disabled(p)) {
if (cpumask_test_cpu(prev_cpu, allowed ?: p->cpus_ptr) &&
scx_idle_test_and_clear_cpu(prev_cpu))
cpu = prev_cpu;