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Subject[PATCH AUTOSEL 5.12 25/80] sched: Make the idle task quack like a per-CPU kthread
Date
From: Valentin Schneider <valentin.schneider@arm.com>

[ Upstream commit 00b89fe0197f0c55a045775c11553c0cdb7082fe ]

For all intents and purposes, the idle task is a per-CPU kthread. It isn't
created via the same route as other pcpu kthreads however, and as a result
it is missing a few bells and whistles: it fails kthread_is_per_cpu() and
it doesn't have PF_NO_SETAFFINITY set.

Fix the former by giving the idle task a kthread struct along with the
KTHREAD_IS_PER_CPU flag. This requires some extra iffery as init_idle()
call be called more than once on the same idle task.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210510151024.2448573-2-valentin.schneider@arm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/linux/kthread.h | 2 ++
kernel/kthread.c | 30 ++++++++++++++++++------------
kernel/sched/core.c | 21 +++++++++++++++------
3 files changed, 35 insertions(+), 18 deletions(-)

diff --git a/include/linux/kthread.h b/include/linux/kthread.h
index 2484ed97e72f..d9133d6db308 100644
--- a/include/linux/kthread.h
+++ b/include/linux/kthread.h
@@ -33,6 +33,8 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
unsigned int cpu,
const char *namefmt);

+void set_kthread_struct(struct task_struct *p);
+
void kthread_set_per_cpu(struct task_struct *k, int cpu);
bool kthread_is_per_cpu(struct task_struct *k);

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 6d3c488a0f82..77dd0ea1d35d 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -68,16 +68,6 @@ enum KTHREAD_BITS {
KTHREAD_SHOULD_PARK,
};

-static inline void set_kthread_struct(void *kthread)
-{
- /*
- * We abuse ->set_child_tid to avoid the new member and because it
- * can't be wrongly copied by copy_process(). We also rely on fact
- * that the caller can't exec, so PF_KTHREAD can't be cleared.
- */
- current->set_child_tid = (__force void __user *)kthread;
-}
-
static inline struct kthread *to_kthread(struct task_struct *k)
{
WARN_ON(!(k->flags & PF_KTHREAD));
@@ -103,6 +93,22 @@ static inline struct kthread *__to_kthread(struct task_struct *p)
return kthread;
}

+void set_kthread_struct(struct task_struct *p)
+{
+ struct kthread *kthread;
+
+ if (__to_kthread(p))
+ return;
+
+ kthread = kzalloc(sizeof(*kthread), GFP_KERNEL);
+ /*
+ * We abuse ->set_child_tid to avoid the new member and because it
+ * can't be wrongly copied by copy_process(). We also rely on fact
+ * that the caller can't exec, so PF_KTHREAD can't be cleared.
+ */
+ p->set_child_tid = (__force void __user *)kthread;
+}
+
void free_kthread_struct(struct task_struct *k)
{
struct kthread *kthread;
@@ -272,8 +278,8 @@ static int kthread(void *_create)
struct kthread *self;
int ret;

- self = kzalloc(sizeof(*self), GFP_KERNEL);
- set_kthread_struct(self);
+ set_kthread_struct(current);
+ self = to_kthread(current);

/* If user was SIGKILLed, I release the structure. */
done = xchg(&create->done, NULL);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e25b2d8ec18d..17f612045271 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7435,12 +7435,25 @@ void __init init_idle(struct task_struct *idle, int cpu)

__sched_fork(0, idle);

+ /*
+ * The idle task doesn't need the kthread struct to function, but it
+ * is dressed up as a per-CPU kthread and thus needs to play the part
+ * if we want to avoid special-casing it in code that deals with per-CPU
+ * kthreads.
+ */
+ set_kthread_struct(idle);
+
raw_spin_lock_irqsave(&idle->pi_lock, flags);
raw_spin_lock(&rq->lock);

idle->state = TASK_RUNNING;
idle->se.exec_start = sched_clock();
- idle->flags |= PF_IDLE;
+ /*
+ * PF_KTHREAD should already be set at this point; regardless, make it
+ * look like a proper per-CPU kthread.
+ */
+ idle->flags |= PF_IDLE | PF_KTHREAD | PF_NO_SETAFFINITY;
+ kthread_set_per_cpu(idle, cpu);

scs_task_reset(idle);
kasan_unpoison_task_stack(idle);
@@ -7647,12 +7660,8 @@ static void balance_push(struct rq *rq)
/*
* Both the cpu-hotplug and stop task are in this case and are
* required to complete the hotplug process.
- *
- * XXX: the idle task does not match kthread_is_per_cpu() due to
- * histerical raisins.
*/
- if (rq->idle == push_task ||
- kthread_is_per_cpu(push_task) ||
+ if (kthread_is_per_cpu(push_task) ||
is_migration_disabled(push_task)) {

/*
--
2.30.2
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