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[RFC][PATCH 02/11] sched: Create preempt_count invariant

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  [RFC][PATCH 02/11] sched: Create preempt_count invariant Peter Zijlstra <peterz@infradead.org> - 2015-09-29 11:50 +0200
    [tip:sched/core] sched/core: Create preempt_count invariant tip-bot for Peter Zijlstra <tipbot@zytor.com> - 2015-09-29 12:40 +0200
    Re: [RFC][PATCH 02/11] sched: Create preempt_count invariant Frederic Weisbecker <fweisbec@gmail.com> - 2015-09-29 15:00 +0200
      Re: [RFC][PATCH 02/11] sched: Create preempt_count invariant Peter Zijlstra <peterz@infradead.org> - 2015-09-29 15:10 +0200
        Re: [RFC][PATCH 02/11] sched: Create preempt_count invariant Steven Rostedt <rostedt@goodmis.org> - 2015-09-29 16:50 +0200
    Re: [RFC][PATCH 02/11] sched: Create preempt_count invariant Thomas Gleixner <tglx@linutronix.de> - 2015-09-29 15:20 +0200
      Re: [RFC][PATCH 02/11] sched: Create preempt_count invariant Peter Zijlstra <peterz@infradead.org> - 2015-09-29 16:20 +0200

#1234928 — [RFC][PATCH 02/11] sched: Create preempt_count invariant

Ensure that upon scheduling preempt_count == 2; although currently an
additional PREEMPT_ACTIVE is still possible.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/include/asm/preempt.h |    3 ++-
 include/asm-generic/preempt.h  |    2 +-
 kernel/sched/core.c            |   14 ++++++++++----
 3 files changed, 13 insertions(+), 6 deletions(-)

--- a/arch/x86/include/asm/preempt.h
+++ b/arch/x86/include/asm/preempt.h
@@ -31,7 +31,8 @@ static __always_inline void preempt_coun
  * must be macros to avoid header recursion hell
  */
 #define init_task_preempt_count(p) do { \
-	task_thread_info(p)->saved_preempt_count = PREEMPT_DISABLED; \
+	task_thread_info(p)->saved_preempt_count = \
+		2*PREEMPT_DISABLE_OFFSET + PREEMPT_NEED_RESCHED; \
 } while (0)
 
 #define init_idle_preempt_count(p, cpu) do { \
--- a/include/asm-generic/preempt.h
+++ b/include/asm-generic/preempt.h
@@ -24,7 +24,7 @@ static __always_inline void preempt_coun
  * must be macros to avoid header recursion hell
  */
 #define init_task_preempt_count(p) do { \
-	task_thread_info(p)->preempt_count = PREEMPT_DISABLED; \
+	task_thread_info(p)->preempt_count = 2*PREEMPT_DISABLED; \
 } while (0)
 
 #define init_idle_preempt_count(p, cpu) do { \
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2588,11 +2588,17 @@ asmlinkage __visible void schedule_tail(
 {
 	struct rq *rq;
 
-	/* finish_task_switch() drops rq->lock and enables preemtion */
-	preempt_disable();
-	rq = finish_task_switch(prev);
+	/*
+	 * Still have preempt_count() == 2, from:
+	 *
+	 *	schedule()
+	 *	  preempt_disable();			// 1
+	 *	  __schedule()
+	 *	    raw_spin_lock_irq(&rq->lock)	// 2
+	 */
+	rq = finish_task_switch(prev); /* drops rq->lock, preempt_count() == 1 */
 	balance_callback(rq);
-	preempt_enable();
+	preempt_enable(); /* preempt_count() == 0 */
 
 	if (current->set_child_tid)
 		put_user(task_pid_vnr(current), current->set_child_tid);


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#1234953 — [tip:sched/core] sched/core: Create preempt_count invariant

Commit-ID:  e26d555f695ac8a3aa38055dd04bd23c1334723b
Gitweb:     http://git.kernel.org/tip/e26d555f695ac8a3aa38055dd04bd23c1334723b
Author:     Peter Zijlstra <peterz@infradead.org>
AuthorDate: Tue, 29 Sep 2015 11:28:27 +0200
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 29 Sep 2015 12:27:40 +0200

sched/core: Create preempt_count invariant

Ensure that upon scheduling preempt_count == 2; although
currently an additional PREEMPT_ACTIVE is still possible.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: fweisbec@gmail.com
Cc: linux-kernel@vger.kernel.org
Cc: oleg@redhat.com
Cc: rostedt@goodmis.org
Cc: umgwanakikbuti@gmail.com
Link: http://lkml.kernel.org/r/20150929093519.817299442@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/include/asm/preempt.h |  3 ++-
 include/asm-generic/preempt.h  |  2 +-
 kernel/sched/core.c            | 14 ++++++++++----
 3 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/preempt.h b/arch/x86/include/asm/preempt.h
index b12f810..183d95c6 100644
--- a/arch/x86/include/asm/preempt.h
+++ b/arch/x86/include/asm/preempt.h
@@ -31,7 +31,8 @@ static __always_inline void preempt_count_set(int pc)
  * must be macros to avoid header recursion hell
  */
 #define init_task_preempt_count(p) do { \
-	task_thread_info(p)->saved_preempt_count = PREEMPT_DISABLED; \
+	task_thread_info(p)->saved_preempt_count = \
+		2*PREEMPT_DISABLE_OFFSET + PREEMPT_NEED_RESCHED; \
 } while (0)
 
 #define init_idle_preempt_count(p, cpu) do { \
diff --git a/include/asm-generic/preempt.h b/include/asm-generic/preempt.h
index 0bec580..1d6f104 100644
--- a/include/asm-generic/preempt.h
+++ b/include/asm-generic/preempt.h
@@ -24,7 +24,7 @@ static __always_inline void preempt_count_set(int pc)
  * must be macros to avoid header recursion hell
  */
 #define init_task_preempt_count(p) do { \
-	task_thread_info(p)->preempt_count = PREEMPT_DISABLED; \
+	task_thread_info(p)->preempt_count = 2*PREEMPT_DISABLED; \
 } while (0)
 
 #define init_idle_preempt_count(p, cpu) do { \
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a91df61..ecd585c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2588,11 +2588,17 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev)
 {
 	struct rq *rq;
 
-	/* finish_task_switch() drops rq->lock and enables preemtion */
-	preempt_disable();
-	rq = finish_task_switch(prev);
+	/*
+	 * Still have preempt_count() == 2, from:
+	 *
+	 *	schedule()
+	 *	  preempt_disable();			// 1
+	 *	  __schedule()
+	 *	    raw_spin_lock_irq(&rq->lock)	// 2
+	 */
+	rq = finish_task_switch(prev); /* drops rq->lock, preempt_count() == 1 */
 	balance_callback(rq);
-	preempt_enable();
+	preempt_enable(); /* preempt_count() == 0 */
 
 	if (current->set_child_tid)
 		put_user(task_pid_vnr(current), current->set_child_tid);
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#1235036

On Tue, Sep 29, 2015 at 11:28:27AM +0200, Peter Zijlstra wrote:
> Ensure that upon scheduling preempt_count == 2; although currently an
> additional PREEMPT_ACTIVE is still possible.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  arch/x86/include/asm/preempt.h |    3 ++-
>  include/asm-generic/preempt.h  |    2 +-
>  kernel/sched/core.c            |   14 ++++++++++----
>  3 files changed, 13 insertions(+), 6 deletions(-)
> 
> --- a/arch/x86/include/asm/preempt.h
> +++ b/arch/x86/include/asm/preempt.h
> @@ -31,7 +31,8 @@ static __always_inline void preempt_coun
>   * must be macros to avoid header recursion hell
>   */
>  #define init_task_preempt_count(p) do { \
> -	task_thread_info(p)->saved_preempt_count = PREEMPT_DISABLED; \
> +	task_thread_info(p)->saved_preempt_count = \
> +		2*PREEMPT_DISABLE_OFFSET + PREEMPT_NEED_RESCHED; \
>  } while (0)
>  
>  #define init_idle_preempt_count(p, cpu) do { \
> --- a/include/asm-generic/preempt.h
> +++ b/include/asm-generic/preempt.h
> @@ -24,7 +24,7 @@ static __always_inline void preempt_coun
>   * must be macros to avoid header recursion hell
>   */
>  #define init_task_preempt_count(p) do { \
> -	task_thread_info(p)->preempt_count = PREEMPT_DISABLED; \
> +	task_thread_info(p)->preempt_count = 2*PREEMPT_DISABLED; \

Since it's not quite obvious why we use this magic value without looking
at schedule_tail() details, maybe add a little comment? (Just "/* see schedule_tail() */").

>  } while (0)
>  
>  #define init_idle_preempt_count(p, cpu) do { \
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2588,11 +2588,17 @@ asmlinkage __visible void schedule_tail(
>  {
>  	struct rq *rq;
>  
> -	/* finish_task_switch() drops rq->lock and enables preemtion */
> -	preempt_disable();
> -	rq = finish_task_switch(prev);
> +	/*
> +	 * Still have preempt_count() == 2, from:
> +	 *
> +	 *	schedule()
> +	 *	  preempt_disable();			// 1
> +	 *	  __schedule()
> +	 *	    raw_spin_lock_irq(&rq->lock)	// 2
> +	 */

I found that a bit confusing first, because that's a preempt_count()
we actually emulate for a new task. Maybe something like:

+	/*
+	 * New task is init with preempt_count() == 2 because prev task left
+        * us after:
+	 *
+	 *	schedule()
+	 *	  preempt_disable();			// 1
+	 *	  __schedule()
+	 *	    raw_spin_lock_irq(&rq->lock)	// 2
+	 */

> +	rq = finish_task_switch(prev); /* drops rq->lock, preempt_count() == 1 */
>  	balance_callback(rq);
> -	preempt_enable();
> +	preempt_enable(); /* preempt_count() == 0 */

Thanks!
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#1235038

On Tue, Sep 29, 2015 at 02:55:13PM +0200, Frederic Weisbecker wrote:
> On Tue, Sep 29, 2015 at 11:28:27AM +0200, Peter Zijlstra wrote:
> >  #define init_task_preempt_count(p) do { \
> > -	task_thread_info(p)->preempt_count = PREEMPT_DISABLED; \
> > +	task_thread_info(p)->preempt_count = 2*PREEMPT_DISABLED; \
> 
> Since it's not quite obvious why we use this magic value without looking
> at schedule_tail() details, maybe add a little comment? (Just "/* see schedule_tail() */").

Right, I fixed that in 12/11 v2. I'll change that around a bit.

> > +	/*
> > +	 * Still have preempt_count() == 2, from:
> > +	 *
> > +	 *	schedule()
> > +	 *	  preempt_disable();			// 1
> > +	 *	  __schedule()
> > +	 *	    raw_spin_lock_irq(&rq->lock)	// 2
> > +	 */
> 
> I found that a bit confusing first, because that's a preempt_count()
> we actually emulate for a new task. Maybe something like:
> 
> +	/*
> +	 * New task is init with preempt_count() == 2 because prev task left
> +        * us after:
> +	 *
> +	 *	schedule()
> +	 *	  preempt_disable();			// 1
> +	 *	  __schedule()
> +	 *	    raw_spin_lock_irq(&rq->lock)	// 2
> +	 */

I think I'll move the comment to finish_task_switch(), but yes.
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#1235178

On Tue, 29 Sep 2015 15:02:01 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, Sep 29, 2015 at 02:55:13PM +0200, Frederic Weisbecker wrote:
> > On Tue, Sep 29, 2015 at 11:28:27AM +0200, Peter Zijlstra wrote:
> > >  #define init_task_preempt_count(p) do { \
> > > -	task_thread_info(p)->preempt_count = PREEMPT_DISABLED; \
> > > +	task_thread_info(p)->preempt_count = 2*PREEMPT_DISABLED; \
> > 
> > Since it's not quite obvious why we use this magic value without looking
> > at schedule_tail() details, maybe add a little comment? (Just "/* see schedule_tail() */").
> 
> Right, I fixed that in 12/11 v2. I'll change that around a bit.

I'll wait for v2 because this confused me too.

-- Steve
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#1235043

On Tue, 29 Sep 2015, Peter Zijlstra wrote:
> +	/*
> +	 * Still have preempt_count() == 2, from:
> +	 *
> +	 *	schedule()
> +	 *	  preempt_disable();			// 1
> +	 *	  __schedule()
> +	 *	    raw_spin_lock_irq(&rq->lock)	// 2
> +	 */
> +	rq = finish_task_switch(prev); /* drops rq->lock, preempt_count() == 1 */
>  	balance_callback(rq);
> -	preempt_enable();
> +	preempt_enable(); /* preempt_count() == 0 */

Bah. I so hate tail comments. What's wrong with

+	 /* preempt_count() ==> 0 */
	preempt_enable();

Hmm?

	tglx
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#1235147

On Tue, Sep 29, 2015 at 03:11:56PM +0200, Thomas Gleixner wrote:
> On Tue, 29 Sep 2015, Peter Zijlstra wrote:
> > +	/*
> > +	 * Still have preempt_count() == 2, from:
> > +	 *
> > +	 *	schedule()
> > +	 *	  preempt_disable();			// 1
> > +	 *	  __schedule()
> > +	 *	    raw_spin_lock_irq(&rq->lock)	// 2
> > +	 */
> > +	rq = finish_task_switch(prev); /* drops rq->lock, preempt_count() == 1 */
> >  	balance_callback(rq);
> > -	preempt_enable();
> > +	preempt_enable(); /* preempt_count() == 0 */
> 
> Bah. I so hate tail comments. What's wrong with
> 
> +	 /* preempt_count() ==> 0 */
> 	preempt_enable();
> 
> Hmm?

I find the tail comments more readable in this case; clearly I don't
share your hatred :-). But I can change them if you insist.

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