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Subject[PATCH v2] sched/fair: Fix detection of per-CPU kthreads waking a task
Date
select_idle_sibling() has a special case for tasks woken up by a per-CPU
kthread, where the selected CPU is the previous one. However, the current
condition for this exit path is incomplete. A task can wake up from an
interrupt context (e.g. hrtimer), while a per-CPU kthread is running. A
such scenario would spuriously trigger the special case described above.
Also, a recent change made the idle task like a regular per-CPU kthread,
hence making that situation more likely to happen
(is_per_cpu_kthread(swapper) being true now).

Checking for task context makes sure select_idle_sibling() will not
interpret a wake up from any other context as a wake up by a per-CPU
kthread.

Fixes: 52262ee567ad ("sched/fair: Allow a per-CPU kthread waking a task to stack on the same CPU, to fix XFS performance regression")
Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>

---
v1 -> v2:
* is_idle_thread() -> in_task() to also include spurious detection when
current != swapper. (Vincent Guittot)
---

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 945d987246c5..56db4ae85995 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6399,6 +6399,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
* pattern is IO completions.
*/
if (is_per_cpu_kthread(current) &&
+ in_task() &&
prev == smp_processor_id() &&
this_rq()->nr_running <= 1) {
return prev;
--
2.25.1
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