[PATCH 0/2] sched: Address idle task vs pcpu kthread checks

[PATCH 0/2] sched: Address idle task vs pcpu kthread checks

[Valentin Schneider]

Commit

  5ba2ffba13a1 ("sched: Fix CPU hotplug / tighten is_per_cpu_kthread()")

had to special-case the idle task when checking for per-CPU kthreads. This is
due to the idle task not having its own struct kthread, which is where we'd
store KTHREAD_IS_PER_CPU.

From staring at Yejune's recent patch [1], it turns out the idle task is also
missing PF_NO_SETAFFINITY.

Patch 1 cleans this up, patch 2 is Yejune's v1 which depends on it.


Note: I remember seeing some patch(es) from Peter tackling this exact
problem, but I couldn't find them again.

[1]: http://lore.kernel.org/r/1620458722-13026-1-git-send-email-yejunedeng@gmail.com

Cheers,
Valentin

Valentin Schneider (1):
  sched: Make the idle task quack like a per-CPU kthread

Yejune Deng (1):
  lib/smp_processor_id: Use is_percpu_thread() instead of
    nr_cpus_allowed

 include/linux/kthread.h |  2 ++
 kernel/kthread.c        | 30 ++++++++++++++++++------------
 kernel/sched/core.c     | 21 +++++++++++++++------
 lib/smp_processor_id.c  |  6 +-----
 4 files changed, 36 insertions(+), 23 deletions(-)

--
2.25.1

[PATCH 1/2] sched: Make the idle task quack like a per-CPU kthread

[Valentin Schneider]

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>
---
 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 4a0668acd876..5bb720829d93 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7440,12 +7440,25 @@ void 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);
@@ -7662,12 +7675,8 @@ 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)) {
 
 		/*
-- 
2.25.1

[PATCH 2/2] lib/smp_processor_id: Use is_percpu_thread() instead of nr_cpus_allowed

[Valentin Schneider]

From: Yejune Deng <yejune.deng@gmail.com>

is_percpu_thread() more elegantly handles SMP vs UP, and further checks the
presence of PF_NO_SETAFFINITY. This lets us catch cases where
check_preemption_disabled() can race with a concurrent sched_setaffinity().

Signed-off-by: Yejune Deng <yejune.deng@gmail.com>
[Amended changelog]
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 lib/smp_processor_id.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/lib/smp_processor_id.c b/lib/smp_processor_id.c
index 1c1dbd300325..046ac6297c78 100644
--- a/lib/smp_processor_id.c
+++ b/lib/smp_processor_id.c
@@ -19,11 +19,7 @@ unsigned int check_preemption_disabled(const char *what1, const char *what2)
 	if (irqs_disabled())
 		goto out;
 
-	/*
-	 * Kernel threads bound to a single CPU can safely use
-	 * smp_processor_id():
-	 */
-	if (current->nr_cpus_allowed == 1)
+	if (is_percpu_thread())
 		goto out;
 
 #ifdef CONFIG_SMP
-- 
2.25.1

Re: [PATCH 1/2] sched: Make the idle task quack like a per-CPU kthread

[Valentin Schneider]

On 10/05/21 16:10, Valentin Schneider wrote:
> This requires some extra iffery as init_idle()
> call be called more than once on the same idle task.
>

While I'm at it, do we actually still need to suffer through this?

AFAICT the extra calls are due to idle_thread_get() (used in cpuhp) calling
init_idle(). However it looks to me that since

  3bb5d2ee396a ("smp, idle: Allocate idle thread for each possible cpu during boot")

we don't need to do that: we already have a

  for_each_possible_cpu(cpu)
    init_idle(cpu)

issued at init. So can't we "simply" rely on that init-time creation, given
it's done against the possible mask? I think the only thing that might need
doing at later hotplug is making sure the preempt count is right (secondary
startups seem to all prepare the idle task by issuing a preempt_disable()).

Re: [PATCH 1/2] sched: Make the idle task quack like a per-CPU kthread

[Ingo Molnar]

* Valentin Schneider <valentin.schneider@arm.com> wrote:

> On 10/05/21 16:10, Valentin Schneider wrote:
> > This requires some extra iffery as init_idle()
> > call be called more than once on the same idle task.
> >
> 
> While I'm at it, do we actually still need to suffer through this?

No.

> AFAICT the extra calls are due to idle_thread_get() (used in cpuhp) 
> calling init_idle(). However it looks to me that since
> 
>   3bb5d2ee396a ("smp, idle: Allocate idle thread for each possible cpu during boot")
> 
> we don't need to do that: we already have a
> 
>   for_each_possible_cpu(cpu)
>     init_idle(cpu)
> 
> issued at init. So can't we "simply" rely on that init-time creation, 
> given it's done against the possible mask? I think the only thing that 
> might need doing at later hotplug is making sure the preempt count is 
> right (secondary startups seem to all prepare the idle task by issuing a 
> preempt_disable()).

Best-case it works, worst-case we discover an unclean assumption in the 
init sequence and it works after we fix that.

Win-win. :-)

Thanks,

	Ingo

Re: [PATCH 1/2] sched: Make the idle task quack like a per-CPU kthread

[Valentin Schneider]

On 11/05/21 09:32, Ingo Molnar wrote:
> * Valentin Schneider <valentin.schneider@arm.com> wrote:
>> AFAICT the extra calls are due to idle_thread_get() (used in cpuhp)
>> calling init_idle(). However it looks to me that since
>>
>>   3bb5d2ee396a ("smp, idle: Allocate idle thread for each possible cpu during boot")
>>
>> we don't need to do that: we already have a
>>
>>   for_each_possible_cpu(cpu)
>>     init_idle(cpu)
>>
>> issued at init. So can't we "simply" rely on that init-time creation,
>> given it's done against the possible mask? I think the only thing that
>> might need doing at later hotplug is making sure the preempt count is
>> right (secondary startups seem to all prepare the idle task by issuing a
>> preempt_disable()).
>
> Best-case it works, worst-case we discover an unclean assumption in the
> init sequence and it works after we fix that.
>
> Win-win. :-)
>

Well I got something that seems to work, let me it test it some more and
convince myself it isn't completely bonkers and I'll toss it out.

> Thanks,
>
>       Ingo

Re: [PATCH 0/2] sched: Address idle task vs pcpu kthread checks

[Peter Zijlstra]

On Mon, May 10, 2021 at 04:10:22PM +0100, Valentin Schneider wrote:
> Note: I remember seeing some patch(es) from Peter tackling this exact
> problem, but I couldn't find them again.

Found it (by accident), yours is nicer though :-)

> Valentin Schneider (1):
>   sched: Make the idle task quack like a per-CPU kthread
> 
> Yejune Deng (1):
>   lib/smp_processor_id: Use is_percpu_thread() instead of
>     nr_cpus_allowed
> 
>  include/linux/kthread.h |  2 ++
>  kernel/kthread.c        | 30 ++++++++++++++++++------------
>  kernel/sched/core.c     | 21 +++++++++++++++------
>  lib/smp_processor_id.c  |  6 +-----
>  4 files changed, 36 insertions(+), 23 deletions(-)

Thanks!

[tip: sched/core] lib/smp_processor_id: Use is_percpu_thread() instead of nr_cpus_allowed

[tip-bot2 for Yejune Deng]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     0019699518cc026b5bd912425be8e424843d5b33
Gitweb:        https://git.kernel.org/tip/0019699518cc026b5bd912425be8e424843d5b33
Author:        Yejune Deng <yejune.deng@gmail.com>
AuthorDate:    Mon, 10 May 2021 16:10:24 +01:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 18 May 2021 12:53:54 +02:00

lib/smp_processor_id: Use is_percpu_thread() instead of nr_cpus_allowed

is_percpu_thread() more elegantly handles SMP vs UP, and further checks the
presence of PF_NO_SETAFFINITY. This lets us catch cases where
check_preemption_disabled() can race with a concurrent sched_setaffinity().

[Amended changelog]
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Yejune Deng <yejune.deng@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210510151024.2448573-3-valentin.schneider@arm.com
---
 lib/smp_processor_id.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/lib/smp_processor_id.c b/lib/smp_processor_id.c
index 1c1dbd3..046ac62 100644
--- a/lib/smp_processor_id.c
+++ b/lib/smp_processor_id.c
@@ -19,11 +19,7 @@ unsigned int check_preemption_disabled(const char *what1, const char *what2)
 	if (irqs_disabled())
 		goto out;
 
-	/*
-	 * Kernel threads bound to a single CPU can safely use
-	 * smp_processor_id():
-	 */
-	if (current->nr_cpus_allowed == 1)
+	if (is_percpu_thread())
 		goto out;
 
 #ifdef CONFIG_SMP

[tip: sched/core] sched: Make the idle task quack like a per-CPU kthread

[tip-bot2 for Valentin Schneider]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     00b89fe0197f0c55a045775c11553c0cdb7082fe
Gitweb:        https://git.kernel.org/tip/00b89fe0197f0c55a045775c11553c0cdb7082fe
Author:        Valentin Schneider <valentin.schneider@arm.com>
AuthorDate:    Mon, 10 May 2021 16:10:23 +01:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 18 May 2021 12:53:53 +02:00

sched: Make the idle task quack like a per-CPU kthread

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
---
 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 2484ed9..d9133d6 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 fe3f2a4..3d32683 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 24fd795..6a5124c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8234,12 +8234,25 @@ 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_rq_lock(rq);
 
 	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);
@@ -8456,12 +8469,8 @@ 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)) {
 
 		/*

[tip: sched/core] lib/smp_processor_id: Use is_percpu_thread() instead of nr_cpus_allowed

[tip-bot2 for Yejune Deng]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     570a752b7a9bd03b50ad6420cd7f10092cc11bd3
Gitweb:        https://git.kernel.org/tip/570a752b7a9bd03b50ad6420cd7f10092cc11bd3
Author:        Yejune Deng <yejune.deng@gmail.com>
AuthorDate:    Mon, 10 May 2021 16:10:24 +01:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 19 May 2021 10:51:40 +02:00

lib/smp_processor_id: Use is_percpu_thread() instead of nr_cpus_allowed

is_percpu_thread() more elegantly handles SMP vs UP, and further checks the
presence of PF_NO_SETAFFINITY. This lets us catch cases where
check_preemption_disabled() can race with a concurrent sched_setaffinity().

Signed-off-by: Yejune Deng <yejune.deng@gmail.com>
[Amended changelog]
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210510151024.2448573-3-valentin.schneider@arm.com
---
 lib/smp_processor_id.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/lib/smp_processor_id.c b/lib/smp_processor_id.c
index 1c1dbd3..046ac62 100644
--- a/lib/smp_processor_id.c
+++ b/lib/smp_processor_id.c
@@ -19,11 +19,7 @@ unsigned int check_preemption_disabled(const char *what1, const char *what2)
 	if (irqs_disabled())
 		goto out;
 
-	/*
-	 * Kernel threads bound to a single CPU can safely use
-	 * smp_processor_id():
-	 */
-	if (current->nr_cpus_allowed == 1)
+	if (is_percpu_thread())
 		goto out;
 
 #ifdef CONFIG_SMP

[PATCH] sched,init: Fix DEBUG_PREEMPT vs early boot

[Peter Zijlstra]

On Wed, May 19, 2021 at 09:02:34AM -0000, tip-bot2 for Yejune Deng wrote:
> The following commit has been merged into the sched/core branch of tip:
> 
> Commit-ID:     570a752b7a9bd03b50ad6420cd7f10092cc11bd3
> Gitweb:        https://git.kernel.org/tip/570a752b7a9bd03b50ad6420cd7f10092cc11bd3
> Author:        Yejune Deng <yejune.deng@gmail.com>
> AuthorDate:    Mon, 10 May 2021 16:10:24 +01:00
> Committer:     Peter Zijlstra <peterz@infradead.org>
> CommitterDate: Wed, 19 May 2021 10:51:40 +02:00
> 
> lib/smp_processor_id: Use is_percpu_thread() instead of nr_cpus_allowed
> 
> is_percpu_thread() more elegantly handles SMP vs UP, and further checks the
> presence of PF_NO_SETAFFINITY. This lets us catch cases where
> check_preemption_disabled() can race with a concurrent sched_setaffinity().
> 
> Signed-off-by: Yejune Deng <yejune.deng@gmail.com>
> [Amended changelog]
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Link: https://lkml.kernel.org/r/20210510151024.2448573-3-valentin.schneider@arm.com
> ---
>  lib/smp_processor_id.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/lib/smp_processor_id.c b/lib/smp_processor_id.c
> index 1c1dbd3..046ac62 100644
> --- a/lib/smp_processor_id.c
> +++ b/lib/smp_processor_id.c
> @@ -19,11 +19,7 @@ unsigned int check_preemption_disabled(const char *what1, const char *what2)
>  	if (irqs_disabled())
>  		goto out;
>  
> -	/*
> -	 * Kernel threads bound to a single CPU can safely use
> -	 * smp_processor_id():
> -	 */
> -	if (current->nr_cpus_allowed == 1)
> +	if (is_percpu_thread())
>  		goto out;

So my test box was unhappy with all this and started spewing lots of
DEBUG_PREEMPT warns on boot.

This extends 8fb12156b8db6 to cover the new requirement.

---
Subject: sched,init: Fix DEBUG_PREEMPT vs early boot

Extend 8fb12156b8db ("init: Pin init task to the boot CPU, initially")
to cover the new PF_NO_SETAFFINITY requirement.

While there, move wait_for_completion(&kthreadd_done) into kernel_init()
to make it absolutely clear it is the very first thing done by the init
thread.

Fixes: 570a752b7a9b ("lib/smp_processor_id: Use is_percpu_thread() instead of nr_cpus_allowed")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 init/main.c         | 11 ++++++-----
 kernel/sched/core.c |  1 +
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/init/main.c b/init/main.c
index 7b027d9c5c89..e945ec82b8a5 100644
--- a/init/main.c
+++ b/init/main.c
@@ -692,6 +692,7 @@ noinline void __ref rest_init(void)
 	 */
 	rcu_read_lock();
 	tsk = find_task_by_pid_ns(pid, &init_pid_ns);
+	tsk->flags |= PF_NO_SETAFFINITY;
 	set_cpus_allowed_ptr(tsk, cpumask_of(smp_processor_id()));
 	rcu_read_unlock();
 
@@ -1440,6 +1441,11 @@ static int __ref kernel_init(void *unused)
 {
 	int ret;
 
+	/*
+	 * Wait until kthreadd is all set-up.
+	 */
+	wait_for_completion(&kthreadd_done);
+
 	kernel_init_freeable();
 	/* need to finish all async __init code before freeing the memory */
 	async_synchronize_full();
@@ -1520,11 +1526,6 @@ void __init console_on_rootfs(void)
 
 static noinline void __init kernel_init_freeable(void)
 {
-	/*
-	 * Wait until kthreadd is all set-up.
-	 */
-	wait_for_completion(&kthreadd_done);
-
 	/* Now the scheduler is fully set up and can do blocking allocations */
 	gfp_allowed_mask = __GFP_BITS_MASK;
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index adea0b1e8036..ae7737e6c2b2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8867,6 +8867,7 @@ void __init sched_init_smp(void)
 	/* Move init over to a non-isolated CPU */
 	if (set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_DOMAIN)) < 0)
 		BUG();
+	current->flags &= ~PF_NO_SETAFFINITY;
 	sched_init_granularity();
 
 	init_sched_rt_class();

Re: [PATCH] sched,init: Fix DEBUG_PREEMPT vs early boot

[Valentin Schneider]

On 31/05/21 12:21, Peter Zijlstra wrote:
> On Wed, May 19, 2021 at 09:02:34AM -0000, tip-bot2 for Yejune Deng wrote:
>> @@ -19,11 +19,7 @@ unsigned int check_preemption_disabled(const char *what1, const char *what2)
>>      if (irqs_disabled())
>>              goto out;
>>
>> -	/*
>> -	 * Kernel threads bound to a single CPU can safely use
>> -	 * smp_processor_id():
>> -	 */
>> -	if (current->nr_cpus_allowed == 1)
>> +	if (is_percpu_thread())
>>              goto out;
>
> So my test box was unhappy with all this and started spewing lots of
> DEBUG_PREEMPT warns on boot.
>

I get these too, though can't recall getting them when testing the
above. I think it's tied with what Frederic found out with copy_process()
copying PF_NO_SETAFFINITY, which it now no longer does.

> This extends 8fb12156b8db6 to cover the new requirement.
>
> ---
> Subject: sched,init: Fix DEBUG_PREEMPT vs early boot
>
> Extend 8fb12156b8db ("init: Pin init task to the boot CPU, initially")
> to cover the new PF_NO_SETAFFINITY requirement.
>
> While there, move wait_for_completion(&kthreadd_done) into kernel_init()
> to make it absolutely clear it is the very first thing done by the init
> thread.
>
> Fixes: 570a752b7a9b ("lib/smp_processor_id: Use is_percpu_thread() instead of nr_cpus_allowed")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>

[tip: sched/core] sched,init: Fix DEBUG_PREEMPT vs early boot

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     15faafc6b449777a85c0cf82dd8286c293fed4eb
Gitweb:        https://git.kernel.org/tip/15faafc6b449777a85c0cf82dd8286c293fed4eb
Author:        Peter Zijlstra <peterz@infradead.org>
AuthorDate:    Mon, 31 May 2021 12:21:13 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 01 Jun 2021 16:00:11 +02:00

sched,init: Fix DEBUG_PREEMPT vs early boot

Extend 8fb12156b8db ("init: Pin init task to the boot CPU, initially")
to cover the new PF_NO_SETAFFINITY requirement.

While there, move wait_for_completion(&kthreadd_done) into kernel_init()
to make it absolutely clear it is the very first thing done by the init
thread.

Fixes: 570a752b7a9b ("lib/smp_processor_id: Use is_percpu_thread() instead of nr_cpus_allowed")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Borislav Petkov <bp@alien8.de>
Link: https://lkml.kernel.org/r/YLS4mbKUrA3Gnb4t@hirez.programming.kicks-ass.net
---
 init/main.c         | 11 ++++++-----
 kernel/sched/core.c |  1 +
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/init/main.c b/init/main.c
index 7b027d9..e945ec8 100644
--- a/init/main.c
+++ b/init/main.c
@@ -692,6 +692,7 @@ noinline void __ref rest_init(void)
 	 */
 	rcu_read_lock();
 	tsk = find_task_by_pid_ns(pid, &init_pid_ns);
+	tsk->flags |= PF_NO_SETAFFINITY;
 	set_cpus_allowed_ptr(tsk, cpumask_of(smp_processor_id()));
 	rcu_read_unlock();
 
@@ -1440,6 +1441,11 @@ static int __ref kernel_init(void *unused)
 {
 	int ret;
 
+	/*
+	 * Wait until kthreadd is all set-up.
+	 */
+	wait_for_completion(&kthreadd_done);
+
 	kernel_init_freeable();
 	/* need to finish all async __init code before freeing the memory */
 	async_synchronize_full();
@@ -1520,11 +1526,6 @@ void __init console_on_rootfs(void)
 
 static noinline void __init kernel_init_freeable(void)
 {
-	/*
-	 * Wait until kthreadd is all set-up.
-	 */
-	wait_for_completion(&kthreadd_done);
-
 	/* Now the scheduler is fully set up and can do blocking allocations */
 	gfp_allowed_mask = __GFP_BITS_MASK;
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3d25272..e205c19 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8862,6 +8862,7 @@ void __init sched_init_smp(void)
 	/* Move init over to a non-isolated CPU */
 	if (set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_DOMAIN)) < 0)
 		BUG();
+	current->flags &= ~PF_NO_SETAFFINITY;
 	sched_init_granularity();
 
 	init_sched_rt_class();