[patch 0/2] sched: Cleanup idle task double initialization

[patch 0/2] sched: Cleanup idle task double initialization

[Thomas Gleixner]

Idle tasks are initialized via __sched_fork() twice, which can be trivially
avoided as pointed out here:

    https://lore.kernel.org/all/8734p4ymqj.ffs@tglx

As this got ignored, the tasteless hack in init_scx_entity() to work around
it stayed and found it's way into mainline.

Mop it up after the fact to not further proliferate technical debt.

Thanks,

	tglx
---
 core.c |   13 ++++---------
 ext.c  |    7 +------
 2 files changed, 5 insertions(+), 15 deletions(-)

[patch 1/2] sched: Initialize idle tasks only once

[Thomas Gleixner]

Idle tasks are initialized via __sched_fork() twice:

     fork_idle()
        copy_process()
	  sched_fork()
             __sched_fork()
	init_idle()
          __sched_fork()
  
Instead of cleaning this up, sched_ext hacked around it. Even when analyis
and solution were provided in a discussion, nobody cared to clean this up.

init_idle() is also invoked from sched_init() to initialize the boot CPU's
idle task, which requires the __sched_fork() invocation. But this can be
trivially solved by invoking __sched_fork() before init_idle() in
sched_init() and removing the __sched_fork() invocation from init_idle().

Do so and clean up the comments explaining this historical leftover.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/8734p4ymqj.ffs@tglx
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Valentin Schneider <vschneid@redhat.com>
---
 kernel/sched/core.c |   12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4424,7 +4424,8 @@ int wake_up_state(struct task_struct *p,
  * Perform scheduler related setup for a newly forked process p.
  * p is forked by current.
  *
- * __sched_fork() is basic setup used by init_idle() too:
+ * __sched_fork() is basic setup which is also used by sched_init() to
+ * initialize the boot CPU's idle task.
  */
 static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
 {
@@ -7680,8 +7681,6 @@ void __init init_idle(struct task_struct
 	struct rq *rq = cpu_rq(cpu);
 	unsigned long flags;
 
-	__sched_fork(0, idle);
-
 	raw_spin_lock_irqsave(&idle->pi_lock, flags);
 	raw_spin_rq_lock(rq);
 
@@ -7696,10 +7695,8 @@ void __init init_idle(struct task_struct
 
 #ifdef CONFIG_SMP
 	/*
-	 * It's possible that init_idle() gets called multiple times on a task,
-	 * in that case do_set_cpus_allowed() will not do the right thing.
-	 *
-	 * And since this is boot we can forgo the serialization.
+	 * No validation and serialization required at boot time and for
+	 * setting up the idle tasks of not yet online CPUs.
 	 */
 	set_cpus_allowed_common(idle, &ac);
 #endif
@@ -8543,6 +8540,7 @@ void __init sched_init(void)
 	 * but because we are the idle thread, we just pick up running again
 	 * when this runqueue becomes "idle".
 	 */
+	__sched_fork(0, current);
 	init_idle(current, smp_processor_id());
 
 	calc_load_update = jiffies + LOAD_FREQ;

[patch 2/2] sched/ext: Remove sched_fork() hack

[Thomas Gleixner]

Instead of solving the underlying problem of the double invocation of
__sched_fork() for idle tasks, sched-ext decided to hack around the issue
by partially clearing out the entity struct to preserve the already
enqueued node. A provided analysis and solution has been ignored for four
months.

Now that someone else has taken care of cleaning it up, remove the
disgusting hack and clear out the full structure.

Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Tejun Heo <tj@kernel.org>
Cc: David Vernet <void@manifault.com>
Cc: Ingo Molnar <mingo@kernel.org.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Valentin Schneider <vschneid@redhat.com>
---
 kernel/sched/ext.c |    7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3548,12 +3548,7 @@ static void scx_ops_exit_task(struct tas
 
 void init_scx_entity(struct sched_ext_entity *scx)
 {
-	/*
-	 * init_idle() calls this function again after fork sequence is
-	 * complete. Don't touch ->tasks_node as it's already linked.
-	 */
-	memset(scx, 0, offsetof(struct sched_ext_entity, tasks_node));
-
+	memset(scx, 0, sizeof(*scx));
 	INIT_LIST_HEAD(&scx->dsq_list.node);
 	RB_CLEAR_NODE(&scx->dsq_priq);
 	scx->sticky_cpu = -1;

Re: [patch 2/2] sched/ext: Remove sched_fork() hack

[Rasmus Villemoes]

On Mon, Oct 28 2024, Thomas Gleixner <tglx@linutronix.de> wrote:

> Instead of solving the underlying problem of the double invocation of
> __sched_fork() for idle tasks, sched-ext decided to hack around the issue
> by partially clearing out the entity struct to preserve the already
> enqueued node. A provided analysis and solution has been ignored for four
> months.
>
> Now that someone else has taken care of cleaning it up, remove the
> disgusting hack and clear out the full structure.
>
> Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: David Vernet <void@manifault.com>
> Cc: Ingo Molnar <mingo@kernel.org.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ben Segall <bsegall@google.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Valentin Schneider <vschneid@redhat.com>
> ---
>  kernel/sched/ext.c |    7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
>
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -3548,12 +3548,7 @@ static void scx_ops_exit_task(struct tas
>  
>  void init_scx_entity(struct sched_ext_entity *scx)
>  {
> -	/*
> -	 * init_idle() calls this function again after fork sequence is
> -	 * complete. Don't touch ->tasks_node as it's already linked.
> -	 */
> -	memset(scx, 0, offsetof(struct sched_ext_entity, tasks_node));
> -
> +	memset(scx, 0, sizeof(*scx));
>  	INIT_LIST_HEAD(&scx->dsq_list.node);
>  	RB_CLEAR_NODE(&scx->dsq_priq);
>  	scx->sticky_cpu = -1;

Should the "must be the last" comment in include/linux/sched/ext.h also
be removed?

Rasmus

Re: [patch 2/2] sched/ext: Remove sched_fork() hack

[Thomas Gleixner]

On Mon, Oct 28 2024 at 13:30, Rasmus Villemoes wrote:
>> +	memset(scx, 0, sizeof(*scx));
>>  	INIT_LIST_HEAD(&scx->dsq_list.node);
>>  	RB_CLEAR_NODE(&scx->dsq_priq);
>>  	scx->sticky_cpu = -1;
>
> Should the "must be the last" comment in include/linux/sched/ext.h also
> be removed?

Oh. Indeed. I missed that one.

Thanks for pointing it out.

       tglx

[patch v1A 2/2] sched/ext: Remove sched_fork() hack

[Thomas Gleixner]

Instead of solving the underlying problem of the double invocation of
__sched_fork() for idle tasks, sched-ext decided to hack around the issue
by partially clearing out the entity struct to preserve the already
enqueued node. A provided analysis and solution has been ignored for four
months.

Now that someone else has taken care of cleaning it up, remove the
disgusting hack and clear out the full structure. Remove the comment in the
structure declaration as well, as there is no requirement for @node being
the last element anymore.

Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Tejun Heo <tj@kernel.org>
Cc: David Vernet <void@manifault.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Valentin Schneider <vschneid@redhat.com>
---
V2: Remove the comment in ext.h  (Rasmus)
---
 include/linux/sched/ext.h |    1 -
 kernel/sched/ext.c        |    7 +------
 2 files changed, 1 insertion(+), 7 deletions(-)

--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -199,7 +199,6 @@ struct sched_ext_entity {
 #ifdef CONFIG_EXT_GROUP_SCHED
 	struct cgroup		*cgrp_moving_from;
 #endif
-	/* must be the last field, see init_scx_entity() */
 	struct list_head	tasks_node;
 };
 
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3548,12 +3548,7 @@ static void scx_ops_exit_task(struct tas
 
 void init_scx_entity(struct sched_ext_entity *scx)
 {
-	/*
-	 * init_idle() calls this function again after fork sequence is
-	 * complete. Don't touch ->tasks_node as it's already linked.
-	 */
-	memset(scx, 0, offsetof(struct sched_ext_entity, tasks_node));
-
+	memset(scx, 0, sizeof(*scx));
 	INIT_LIST_HEAD(&scx->dsq_list.node);
 	RB_CLEAR_NODE(&scx->dsq_priq);
 	scx->sticky_cpu = -1;

Re: [patch v1A 2/2] sched/ext: Remove sched_fork() hack

[Tejun Heo]

On Mon, Oct 28, 2024 at 02:20:35PM +0100, Thomas Gleixner wrote:
> Instead of solving the underlying problem of the double invocation of
> __sched_fork() for idle tasks, sched-ext decided to hack around the issue
> by partially clearing out the entity struct to preserve the already
> enqueued node. A provided analysis and solution has been ignored for four
> months.
> 
> Now that someone else has taken care of cleaning it up, remove the
> disgusting hack and clear out the full structure. Remove the comment in the
> structure declaration as well, as there is no requirement for @node being
> the last element anymore.
> 
> Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: David Vernet <void@manifault.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ben Segall <bsegall@google.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Valentin Schneider <vschneid@redhat.com>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun

[tip: sched/core] sched/ext: Remove sched_fork() hack

[tip-bot2 for Thomas Gleixner]

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

Commit-ID:     0f0d1b8e5010bfe1feeb4d78d137e41946a5370d
Gitweb:        https://git.kernel.org/tip/0f0d1b8e5010bfe1feeb4d78d137e41946a5370d
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Mon, 28 Oct 2024 14:20:35 +01:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 05 Nov 2024 12:55:37 +01:00

sched/ext: Remove sched_fork() hack

Instead of solving the underlying problem of the double invocation of
__sched_fork() for idle tasks, sched-ext decided to hack around the issue
by partially clearing out the entity struct to preserve the already
enqueued node. A provided analysis and solution has been ignored for four
months.

Now that someone else has taken care of cleaning it up, remove the
disgusting hack and clear out the full structure. Remove the comment in the
structure declaration as well, as there is no requirement for @node being
the last element anymore.

Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/87ldy82wkc.ffs@tglx
---
 include/linux/sched/ext.h | 1 -
 kernel/sched/ext.c        | 7 +------
 2 files changed, 1 insertion(+), 7 deletions(-)

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index 1ddbde6..2799e72 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -199,7 +199,6 @@ struct sched_ext_entity {
 #ifdef CONFIG_EXT_GROUP_SCHED
 	struct cgroup		*cgrp_moving_from;
 #endif
-	/* must be the last field, see init_scx_entity() */
 	struct list_head	tasks_node;
 };
 
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 5900b06..f6e9a14 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3548,12 +3548,7 @@ static void scx_ops_exit_task(struct task_struct *p)
 
 void init_scx_entity(struct sched_ext_entity *scx)
 {
-	/*
-	 * init_idle() calls this function again after fork sequence is
-	 * complete. Don't touch ->tasks_node as it's already linked.
-	 */
-	memset(scx, 0, offsetof(struct sched_ext_entity, tasks_node));
-
+	memset(scx, 0, sizeof(*scx));
 	INIT_LIST_HEAD(&scx->dsq_list.node);
 	RB_CLEAR_NODE(&scx->dsq_priq);
 	scx->sticky_cpu = -1;

[tip: sched/core] sched: Initialize idle tasks only once

[tip-bot2 for Thomas Gleixner]

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

Commit-ID:     b23decf8ac9102fc52c4de5196f4dc0a5f3eb80b
Gitweb:        https://git.kernel.org/tip/b23decf8ac9102fc52c4de5196f4dc0a5f3eb80b
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Mon, 28 Oct 2024 11:43:42 +01:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 05 Nov 2024 12:55:37 +01:00

sched: Initialize idle tasks only once

Idle tasks are initialized via __sched_fork() twice:

     fork_idle()
        copy_process()
	  sched_fork()
             __sched_fork()
	init_idle()
          __sched_fork()

Instead of cleaning this up, sched_ext hacked around it. Even when analyis
and solution were provided in a discussion, nobody cared to clean this up.

init_idle() is also invoked from sched_init() to initialize the boot CPU's
idle task, which requires the __sched_fork() invocation. But this can be
trivially solved by invoking __sched_fork() before init_idle() in
sched_init() and removing the __sched_fork() invocation from init_idle().

Do so and clean up the comments explaining this historical leftover.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241028103142.359584747@linutronix.de
---
 kernel/sched/core.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c57a79e..aad4885 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4423,7 +4423,8 @@ int wake_up_state(struct task_struct *p, unsigned int state)
  * Perform scheduler related setup for a newly forked process p.
  * p is forked by current.
  *
- * __sched_fork() is basic setup used by init_idle() too:
+ * __sched_fork() is basic setup which is also used by sched_init() to
+ * initialize the boot CPU's idle task.
  */
 static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
 {
@@ -7697,8 +7698,6 @@ void __init init_idle(struct task_struct *idle, int cpu)
 	struct rq *rq = cpu_rq(cpu);
 	unsigned long flags;
 
-	__sched_fork(0, idle);
-
 	raw_spin_lock_irqsave(&idle->pi_lock, flags);
 	raw_spin_rq_lock(rq);
 
@@ -7713,10 +7712,8 @@ void __init init_idle(struct task_struct *idle, int cpu)
 
 #ifdef CONFIG_SMP
 	/*
-	 * It's possible that init_idle() gets called multiple times on a task,
-	 * in that case do_set_cpus_allowed() will not do the right thing.
-	 *
-	 * And since this is boot we can forgo the serialization.
+	 * No validation and serialization required at boot time and for
+	 * setting up the idle tasks of not yet online CPUs.
 	 */
 	set_cpus_allowed_common(idle, &ac);
 #endif
@@ -8561,6 +8558,7 @@ void __init sched_init(void)
 	 * but because we are the idle thread, we just pick up running again
 	 * when this runqueue becomes "idle".
 	 */
+	__sched_fork(0, current);
 	init_idle(current, smp_processor_id());
 
 	calc_load_update = jiffies + LOAD_FREQ;