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[PATCH v2 00/12] sched: Killing PREEMPT_ACTIVE

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  [PATCH v2 00/12] sched: Killing PREEMPT_ACTIVE Peter Zijlstra <peterz@infradead.org> - 2015-09-30 09:40 +0200
    [PATCH v2 04/12] sched: Add preempt argument to __schedule() Peter Zijlstra <peterz@infradead.org> - 2015-09-30 09:40 +0200
    [PATCH v2 06/12] sched: Stop setting PREEMPT_ACTIVE Peter Zijlstra <peterz@infradead.org> - 2015-09-30 09:40 +0200
      Re: [PATCH v2 06/12] sched: Stop setting PREEMPT_ACTIVE Frederic Weisbecker <fweisbec@gmail.com> - 2015-10-01 17:30 +0200
    [PATCH v2 08/12] sched: Simplify preempt_count tests Peter Zijlstra <peterz@infradead.org> - 2015-09-30 09:40 +0200
      Re: [PATCH v2 08/12] sched: Simplify preempt_count tests Frederic Weisbecker <fweisbec@gmail.com> - 2015-10-01 17:40 +0200
    [PATCH v2 12/12] sched: Add preempt_count invariant check Peter Zijlstra <peterz@infradead.org> - 2015-09-30 09:40 +0200
      Re: [PATCH v2 12/12] sched: Add preempt_count invariant check Steven Rostedt <rostedt@goodmis.org> - 2015-09-30 11:40 +0200
        Re: [PATCH v2 12/12] sched: Add preempt_count invariant check Peter Zijlstra <peterz@infradead.org> - 2015-09-30 13:20 +0200
    [PATCH v2 11/12] sched: More notrace Peter Zijlstra <peterz@infradead.org> - 2015-09-30 09:40 +0200
    [PATCH v2 01/12] sched: Simplify INIT_PREEMPT_COUNT Peter Zijlstra <peterz@infradead.org> - 2015-09-30 09:40 +0200
      Re: [PATCH v2 01/12] sched: Simplify INIT_PREEMPT_COUNT Steven Rostedt <rostedt@goodmis.org> - 2015-09-30 11:10 +0200
        Re: [PATCH v2 01/12] sched: Simplify INIT_PREEMPT_COUNT Thomas Gleixner <tglx@linutronix.de> - 2015-09-30 22:50 +0200
      Re: [PATCH v2 01/12] sched: Simplify INIT_PREEMPT_COUNT Frederic Weisbecker <fweisbec@gmail.com> - 2015-10-01 15:30 +0200

#1235843 — [PATCH v2 00/12] sched: Killing PREEMPT_ACTIVE

Second posting of the series that kills PREEMPT_ACTIVE dead. Thanks for all the
feedback.

I've re-oredered the patches slightly and reworked one or two; those that got
significant changes I've ignored the Reviewed-by tags for.

Please have another careful look.

---
 arch/x86/include/asm/preempt.h     |  5 +--
 arch/x86/include/asm/thread_info.h |  2 -
 arch/x86/kernel/process_32.c       |  8 ----
 arch/x86/kernel/process_64.c       |  8 ----
 include/asm-generic/preempt.h      |  2 +-
 include/linux/preempt.h            | 20 +--------
 include/linux/sched.h              | 26 +++++++-----
 include/trace/events/sched.h       | 22 +++++-----
 kernel/exit.c                      |  4 +-
 kernel/sched/core.c                | 83 +++++++++++++++++++++++++-------------
 kernel/trace/ftrace.c              |  2 +-
 kernel/trace/trace_sched_switch.c  |  3 +-
 kernel/trace/trace_sched_wakeup.c  |  2 +-
 13 files changed, 89 insertions(+), 98 deletions(-)

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#1235844 — [PATCH v2 04/12] sched: Add preempt argument to __schedule()

There is only a single PREEMPT_ACTIVE use in the regular __schedule()
path and that is to circumvent the task->state check. Since the code
setting PREEMPT_ACTIVE is the immediate caller of __schedule() we can
replace this with a function argument.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/sched/core.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3037,7 +3037,7 @@ pick_next_task(struct rq *rq, struct tas
  *
  * WARNING: must be called with preemption disabled!
  */
-static void __sched __schedule(void)
+static void __sched __schedule(bool preempt)
 {
 	struct task_struct *prev, *next;
 	unsigned long *switch_count;
@@ -3077,7 +3077,7 @@ static void __sched __schedule(void)
 	rq->clock_skip_update <<= 1; /* promote REQ to ACT */
 
 	switch_count = &prev->nivcsw;
-	if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
+	if (!preempt && prev->state) {
 		if (unlikely(signal_pending_state(prev->state, prev))) {
 			prev->state = TASK_RUNNING;
 		} else {
@@ -3142,7 +3142,7 @@ asmlinkage __visible void __sched schedu
 	sched_submit_work(tsk);
 	do {
 		preempt_disable();
-		__schedule();
+		__schedule(false);
 		sched_preempt_enable_no_resched();
 	} while (need_resched());
 }
@@ -3183,7 +3183,7 @@ static void __sched notrace preempt_sche
 {
 	do {
 		preempt_active_enter();
-		__schedule();
+		__schedule(true);
 		preempt_active_exit();
 
 		/*
@@ -3248,7 +3248,7 @@ asmlinkage __visible void __sched notrac
 		 * an infinite recursion.
 		 */
 		prev_ctx = exception_enter();
-		__schedule();
+		__schedule(true);
 		exception_exit(prev_ctx);
 
 		barrier();
@@ -3277,7 +3277,7 @@ asmlinkage __visible void __sched preemp
 	do {
 		preempt_active_enter();
 		local_irq_enable();
-		__schedule();
+		__schedule(true);
 		local_irq_disable();
 		preempt_active_exit();
 	} while (need_resched());


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#1235845 — [PATCH v2 06/12] sched: Stop setting PREEMPT_ACTIVE

Now that nothing tests for PREEMPT_ACTIVE anymore, stop setting it.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/preempt.h |   12 ------------
 kernel/sched/core.c     |   19 ++++++-------------
 2 files changed, 6 insertions(+), 25 deletions(-)

--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -146,18 +146,6 @@ extern void preempt_count_sub(int val);
 #define preempt_count_inc() preempt_count_add(1)
 #define preempt_count_dec() preempt_count_sub(1)
 
-#define preempt_active_enter() \
-do { \
-	preempt_count_add(PREEMPT_ACTIVE + PREEMPT_DISABLE_OFFSET); \
-	barrier(); \
-} while (0)
-
-#define preempt_active_exit() \
-do { \
-	barrier(); \
-	preempt_count_sub(PREEMPT_ACTIVE + PREEMPT_DISABLE_OFFSET); \
-} while (0)
-
 #ifdef CONFIG_PREEMPT_COUNT
 
 #define preempt_disable() \
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3190,9 +3190,9 @@ void __sched schedule_preempt_disabled(v
 static void __sched notrace preempt_schedule_common(void)
 {
 	do {
-		preempt_active_enter();
+		preempt_disable();
 		__schedule(true);
-		preempt_active_exit();
+		sched_preempt_enable_no_resched();
 
 		/*
 		 * Check again in case we missed a preemption opportunity
@@ -3243,13 +3243,7 @@ asmlinkage __visible void __sched notrac
 		return;
 
 	do {
-		/*
-		 * Use raw __prempt_count() ops that don't call function.
-		 * We can't call functions before disabling preemption which
-		 * disarm preemption tracing recursions.
-		 */
-		__preempt_count_add(PREEMPT_ACTIVE + PREEMPT_DISABLE_OFFSET);
-		barrier();
+		preempt_disable_notrace();
 		/*
 		 * Needs preempt disabled in case user_exit() is traced
 		 * and the tracer calls preempt_enable_notrace() causing
@@ -3259,8 +3253,7 @@ asmlinkage __visible void __sched notrac
 		__schedule(true);
 		exception_exit(prev_ctx);
 
-		barrier();
-		__preempt_count_sub(PREEMPT_ACTIVE + PREEMPT_DISABLE_OFFSET);
+		preempt_enable_no_resched_notrace();
 	} while (need_resched());
 }
 EXPORT_SYMBOL_GPL(preempt_schedule_notrace);
@@ -3283,11 +3276,11 @@ asmlinkage __visible void __sched preemp
 	prev_state = exception_enter();
 
 	do {
-		preempt_active_enter();
+		preempt_disable();
 		local_irq_enable();
 		__schedule(true);
 		local_irq_disable();
-		preempt_active_exit();
+		sched_preempt_enable_no_resched();
 	} while (need_resched());
 
 	exception_exit(prev_state);


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#1237511 — Re: [PATCH v2 06/12] sched: Stop setting PREEMPT_ACTIVE

On Wed, Sep 30, 2015 at 09:10:41AM +0200, Peter Zijlstra wrote:
> Now that nothing tests for PREEMPT_ACTIVE anymore, stop setting it.
> 
> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
> Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Great news!

Reviewed-by: Frederic Weisbecker <fweisbec@gmail.com>
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#1235847 — [PATCH v2 08/12] sched: Simplify preempt_count tests

Since we stopped setting PREEMPT_ACTIVE, there is no need to mask it
out of preempt_count() tests.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/preempt.h |    3 +--
 kernel/sched/core.c     |    2 +-
 2 files changed, 2 insertions(+), 3 deletions(-)

--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -126,8 +126,7 @@
  * Check whether we were atomic before we did preempt_disable():
  * (used by the scheduler)
  */
-#define in_atomic_preempt_off() \
-		((preempt_count() & ~PREEMPT_ACTIVE) != PREEMPT_DISABLE_OFFSET)
+#define in_atomic_preempt_off() (preempt_count() != PREEMPT_DISABLE_OFFSET)
 
 #if defined(CONFIG_DEBUG_PREEMPT) || defined(CONFIG_PREEMPT_TRACER)
 extern void preempt_count_add(int val);
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7472,7 +7472,7 @@ void __init sched_init(void)
 #ifdef CONFIG_DEBUG_ATOMIC_SLEEP
 static inline int preempt_count_equals(int preempt_offset)
 {
-	int nested = (preempt_count() & ~PREEMPT_ACTIVE) + rcu_preempt_depth();
+	int nested = preempt_count() + rcu_preempt_depth();
 
 	return (nested == preempt_offset);
 }


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#1237517 — Re: [PATCH v2 08/12] sched: Simplify preempt_count tests

On Wed, Sep 30, 2015 at 09:10:43AM +0200, Peter Zijlstra wrote:
> Since we stopped setting PREEMPT_ACTIVE, there is no need to mask it
> out of preempt_count() tests.
> 
> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
> Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Reviewed-by: Frederic Weisbecker <fweisbec@gmail.com>
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#1235848 — [PATCH v2 12/12] sched: Add preempt_count invariant check

Ingo requested I keep my debug check for the preempt_count invariant.

Requested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/sched/core.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2514,6 +2514,10 @@ static struct rq *finish_task_switch(str
 	 *
 	 * Also, see FORK_PREEMPT_COUNT.
 	 */
+	if (unlikely(WARN_ONCE(preempt_count() != 2*PREEMPT_DISABLE_OFFSET,
+			       "corrupted preempt_count: %s/%d/0x%x\n",
+			       current->comm, current->pid, preempt_count())))
+		preempt_count_set(FORK_PREEMPT_COUNT);
 
 	rq->prev_mm = NULL;
 


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#1235938 — Re: [PATCH v2 12/12] sched: Add preempt_count invariant check

On Wed, 30 Sep 2015 09:10:47 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> Ingo requested I keep my debug check for the preempt_count invariant.
> 
> Requested-by: Ingo Molnar <mingo@kernel.org>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  kernel/sched/core.c |    4 ++++
>  1 file changed, 4 insertions(+)
> 
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2514,6 +2514,10 @@ static struct rq *finish_task_switch(str
>  	 *
>  	 * Also, see FORK_PREEMPT_COUNT.
>  	 */
> +	if (unlikely(WARN_ONCE(preempt_count() != 2*PREEMPT_DISABLE_OFFSET,

Nuke the "unlikely" it's redundant with the WARN_ONCE().

-- Steve

> +			       "corrupted preempt_count: %s/%d/0x%x\n",
> +			       current->comm, current->pid, preempt_count())))
> +		preempt_count_set(FORK_PREEMPT_COUNT);
>  
>  	rq->prev_mm = NULL;
>  
> 

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#1236169 — Re: [PATCH v2 12/12] sched: Add preempt_count invariant check

On Wed, Sep 30, 2015 at 05:38:10AM -0400, Steven Rostedt wrote:
> On Wed, 30 Sep 2015 09:10:47 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > Ingo requested I keep my debug check for the preempt_count invariant.
> > 
> > Requested-by: Ingo Molnar <mingo@kernel.org>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > ---
> >  kernel/sched/core.c |    4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -2514,6 +2514,10 @@ static struct rq *finish_task_switch(str
> >  	 *
> >  	 * Also, see FORK_PREEMPT_COUNT.
> >  	 */
> > +	if (unlikely(WARN_ONCE(preempt_count() != 2*PREEMPT_DISABLE_OFFSET,
> 
> Nuke the "unlikely" it's redundant with the WARN_ONCE().

Ah, indeed. *poof*
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#1235849 — [PATCH v2 11/12] sched: More notrace

preempt_schedule_common() is marked notrace, but it does not use
_notrace() preempt_count functions and __schedule() is also not marked
notrace, which means that its perfectly possible to end up in the
tracer from preempt_schedule_common().

Steve says:

| Yep, there's some history to this. This was originally the issue that
| caused function tracing to go into infinite recursion. But now we have
| preempt_schedule_notrace(), which is used by the function tracer, and
| that function must not be traced till preemption is disabled.
|
| Now if function tracing is running and we take an interrupt when
| NEED_RESCHED is set, it calls
|
|   preempt_schedule_common() (not traced)
|
| But then that calls preempt_disable() (traced)
|
| function tracer calls preempt_disable_notrace() followed by
| preempt_enable_notrace() which will see NEED_RESCHED set, and it will
| call preempt_schedule_notrace(), which stops the recursion, but
| still calls __schedule() here, and that means when we return, we call
| the __schedule() from preempt_schedule_common().
|
| That said, I prefer this patch. Preemption is disabled before calling
| __schedule(), and we get rid of a one round recursion with the
| scheduler.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/sched/core.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3057,7 +3057,7 @@ pick_next_task(struct rq *rq, struct tas
  *
  * WARNING: must be called with preemption disabled!
  */
-static void __sched __schedule(bool preempt)
+static void __sched notrace __schedule(bool preempt)
 {
 	struct task_struct *prev, *next;
 	unsigned long *switch_count;
@@ -3203,9 +3203,9 @@ void __sched schedule_preempt_disabled(v
 static void __sched notrace preempt_schedule_common(void)
 {
 	do {
-		preempt_disable();
+		preempt_disable_notrace();
 		__schedule(true);
-		sched_preempt_enable_no_resched();
+		preempt_enable_no_resched_notrace();
 
 		/*
 		 * Check again in case we missed a preemption opportunity


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#1235852 — [PATCH v2 01/12] sched: Simplify INIT_PREEMPT_COUNT

As per commit d86ee4809d03 ("sched: optimize cond_resched()") we need
PREEMPT_ACTIVE to avoid cond_resched() from working before the
scheduler is setup.

However, keeping preemption disabled should do the same thing already,
making the PREEMPT_ACTIVE part entirely redundant.

The only complication is !PREEMPT_COUNT kernels, where
PREEMPT_DISABLED ends up being 0. Instead we use an unconditional
PREEMPT_OFFSET to set preempt_count() even on !PREEMPT_COUNT kernels.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/sched.h |   11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -606,19 +606,18 @@ struct task_cputime_atomic {
 #endif
 
 /*
- * Disable preemption until the scheduler is running.
- * Reset by start_kernel()->sched_init()->init_idle().
+ * Disable preemption until the scheduler is running -- use an unconditional
+ * value so that it also works on !PREEMPT_COUNT kernels.
  *
- * We include PREEMPT_ACTIVE to avoid cond_resched() from working
- * before the scheduler is active -- see should_resched().
+ * Reset by start_kernel()->sched_init()->init_idle().
  */
-#define INIT_PREEMPT_COUNT	(PREEMPT_DISABLED + PREEMPT_ACTIVE)
+#define INIT_PREEMPT_COUNT	PREEMPT_OFFSET
 
 /**
  * struct thread_group_cputimer - thread group interval timer counts
  * @cputime_atomic:	atomic thread group interval timers.
  * @running:		non-zero when there are timers running and
- * 			@cputime receives updates.
+ *			@cputime receives updates.
  *
  * This structure contains the version of task_cputime, above, that is
  * used for thread group CPU timer calculations.


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#1235922 — Re: [PATCH v2 01/12] sched: Simplify INIT_PREEMPT_COUNT

On Wed, 30 Sep 2015 09:10:36 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> As per commit d86ee4809d03 ("sched: optimize cond_resched()") we need
> PREEMPT_ACTIVE to avoid cond_resched() from working before the
> scheduler is setup.
> 
> However, keeping preemption disabled should do the same thing already,
> making the PREEMPT_ACTIVE part entirely redundant.
> 
> The only complication is !PREEMPT_COUNT kernels, where
> PREEMPT_DISABLED ends up being 0. Instead we use an unconditional
> PREEMPT_OFFSET to set preempt_count() even on !PREEMPT_COUNT kernels.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  include/linux/sched.h |   11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
> 
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -606,19 +606,18 @@ struct task_cputime_atomic {
>  #endif
>  
>  /*
> - * Disable preemption until the scheduler is running.
> - * Reset by start_kernel()->sched_init()->init_idle().
> + * Disable preemption until the scheduler is running -- use an unconditional
> + * value so that it also works on !PREEMPT_COUNT kernels.
>   *
> - * We include PREEMPT_ACTIVE to avoid cond_resched() from working
> - * before the scheduler is active -- see should_resched().
> + * Reset by start_kernel()->sched_init()->init_idle().

 Reset by start_kernel()->sched_init()->init_idle()->init_idle_preempt_count().

Other than that.

Reviewed-by: Steven Rostedt <rostedt@goodmis.org>

-- Steve

>   */
> -#define INIT_PREEMPT_COUNT	(PREEMPT_DISABLED + PREEMPT_ACTIVE)
> +#define INIT_PREEMPT_COUNT	PREEMPT_OFFSET
>  
>  /**
>   * struct thread_group_cputimer - thread group interval timer counts
>   * @cputime_atomic:	atomic thread group interval timers.
>   * @running:		non-zero when there are timers running and
> - * 			@cputime receives updates.
> + *			@cputime receives updates.
>   *
>   * This structure contains the version of task_cputime, above, that is
>   * used for thread group CPU timer calculations.
> 

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#1236747 — Re: [PATCH v2 01/12] sched: Simplify INIT_PREEMPT_COUNT

On Wed, 30 Sep 2015, Steven Rostedt wrote:
> >  /*
> > - * Disable preemption until the scheduler is running.
> > - * Reset by start_kernel()->sched_init()->init_idle().
> > + * Disable preemption until the scheduler is running -- use an unconditional
> > + * value so that it also works on !PREEMPT_COUNT kernels.
> >   *
> > - * We include PREEMPT_ACTIVE to avoid cond_resched() from working
> > - * before the scheduler is active -- see should_resched().
> > + * Reset by start_kernel()->sched_init()->init_idle().
> 
>  Reset by start_kernel()->sched_init()->init_idle()->init_idle_preempt_count().
> 
> Other than that.
> 
> Reviewed-by: Steven Rostedt <rostedt@goodmis.org>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
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#1237424 — Re: [PATCH v2 01/12] sched: Simplify INIT_PREEMPT_COUNT

On Wed, Sep 30, 2015 at 09:10:36AM +0200, Peter Zijlstra wrote:
> As per commit d86ee4809d03 ("sched: optimize cond_resched()") we need
> PREEMPT_ACTIVE to avoid cond_resched() from working before the
> scheduler is setup.
> 
> However, keeping preemption disabled should do the same thing already,
> making the PREEMPT_ACTIVE part entirely redundant.
> 
> The only complication is !PREEMPT_COUNT kernels, where
> PREEMPT_DISABLED ends up being 0. Instead we use an unconditional
> PREEMPT_OFFSET to set preempt_count() even on !PREEMPT_COUNT kernels.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Reviewed-by: Frederic Weisbecker <fweisbec@gmail.com>

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