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[RFC][PATCH 01/11] sched: Simplify INIT_PREEMPT_COUNT

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  [RFC][PATCH 01/11] sched: Simplify INIT_PREEMPT_COUNT Peter Zijlstra <peterz@infradead.org> - 2015-09-29 11:50 +0200
    [tip:sched/core] sched/core: Simplify INIT_PREEMPT_COUNT tip-bot for Peter Zijlstra <tipbot@zytor.com> - 2015-09-29 12:40 +0200
    Re: [RFC][PATCH 01/11] sched: Simplify INIT_PREEMPT_COUNT Frederic Weisbecker <fweisbec@gmail.com> - 2015-09-29 15:30 +0200
    Re: [RFC][PATCH 01/11] sched: Simplify INIT_PREEMPT_COUNT Steven Rostedt <rostedt@goodmis.org> - 2015-09-29 16:50 +0200
      Re: [RFC][PATCH 01/11] sched: Simplify INIT_PREEMPT_COUNT Peter Zijlstra <peterz@infradead.org> - 2015-09-29 17:00 +0200
    Re: [RFC][PATCH 01/11] sched: Simplify INIT_PREEMPT_COUNT Steven Rostedt <rostedt@goodmis.org> - 2015-09-29 17:10 +0200
      Re: [RFC][PATCH 01/11] sched: Simplify INIT_PREEMPT_COUNT Peter Zijlstra <peterz@infradead.org> - 2015-09-29 17:20 +0200

#1234925 — [RFC][PATCH 01/11] 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.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/sched.h |    7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -608,17 +608,14 @@ struct task_cputime_atomic {
 /*
  * Disable preemption until the scheduler is running.
  * Reset by start_kernel()->sched_init()->init_idle().
- *
- * We include PREEMPT_ACTIVE to avoid cond_resched() from working
- * before the scheduler is active -- see should_resched().
  */
-#define INIT_PREEMPT_COUNT	(PREEMPT_DISABLED + PREEMPT_ACTIVE)
+#define INIT_PREEMPT_COUNT	PREEMPT_DISABLED
 
 /**
  * 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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#1234956 — [tip:sched/core] sched/core: Simplify INIT_PREEMPT_COUNT

Commit-ID:  aed586c2f11e5cddd52c180ccde651d0d35b736b
Gitweb:     http://git.kernel.org/tip/aed586c2f11e5cddd52c180ccde651d0d35b736b
Author:     Peter Zijlstra <peterz@infradead.org>
AuthorDate: Tue, 29 Sep 2015 11:28:26 +0200
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 29 Sep 2015 12:27:39 +0200

sched/core: 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.

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/20150929093519.706413197@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 include/linux/sched.h | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 699228b..5ef9e2c 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -608,17 +608,14 @@ struct task_cputime_atomic {
 /*
  * Disable preemption until the scheduler is running.
  * Reset by start_kernel()->sched_init()->init_idle().
- *
- * We include PREEMPT_ACTIVE to avoid cond_resched() from working
- * before the scheduler is active -- see should_resched().
  */
-#define INIT_PREEMPT_COUNT	(PREEMPT_DISABLED + PREEMPT_ACTIVE)
+#define INIT_PREEMPT_COUNT	PREEMPT_DISABLED
 
 /**
  * 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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#1235050

On Tue, Sep 29, 2015 at 11:28:26AM +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.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Reviewed-by: Frederic Weisbecker <fweisbec@gmail.com>
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#1235186

[ Resend with a "Reply-all" this time! ]

On Tue, 29 Sep 2015 11:28:26 +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.

Thus PREEMPT_ACTIVE wasn't needed at that commit either, or was it?

> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  include/linux/sched.h |    7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -608,17 +608,14 @@ struct task_cputime_atomic {
>  /*
>   * Disable preemption until the scheduler is running.
>   * Reset by start_kernel()->sched_init()->init_idle().
> - *
> - * We include PREEMPT_ACTIVE to avoid cond_resched() from working
> - * before the scheduler is active -- see should_resched().
>   */
> -#define INIT_PREEMPT_COUNT	(PREEMPT_DISABLED + PREEMPT_ACTIVE)
> +#define INIT_PREEMPT_COUNT	PREEMPT_DISABLED

I can't find anything wrong with this.

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

-- Steve

>  
>  /**
>   * 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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#1235199

On Tue, Sep 29, 2015 at 10:43:32AM -0400, Steven Rostedt wrote:
> 
> [ Resend with a "Reply-all" this time! ]
> 
> On Tue, 29 Sep 2015 11:28:26 +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.
> 
> Thus PREEMPT_ACTIVE wasn't needed at that commit either, or was it?

Correct. We might have actually discussed that at that time but decided
to be conservative -- but these be vague memories..

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#1235206

[ Resend, with the Cc list back ]

On Tue, 29 Sep 2015 16:35:59 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, Sep 29, 2015 at 10:37:29AM -0400, Steven Rostedt wrote:
> > On Tue, 29 Sep 2015 11:28:26 +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.
> > 
> > Thus PREEMPT_ACTIVE wasn't needed at that commit either, or was it?
> 
> Correct. We might have actually discussed that at that time but decided
> to be conservative -- but these be vague memories..

Hmm, I just looked closer and I'm wondering if this still works?

We have:

#ifdef CONFIG_PREEMPT_COUNT
#define PREEMPT_DISABLED	(1 + PREEMPT_ENABLED)
#else
#define PREEMPT_DISABLED	PREEMPT_ENABLED
#endif

Now if we just remove the PREEMPT_ACTIVE from the INIT_PREEMPT_COUNT,
when CONFIG_PREEMPT_COUNT is not set, wouldn't a _cond_resched() in
boot up schedule?

-- Steve
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#1235219

On Tue, Sep 29, 2015 at 11:00:49AM -0400, Steven Rostedt wrote:
> 
> [ Resend, with the Cc list back ]

You seem to have a hard time with these buttons today ;-)

> Hmm, I just looked closer and I'm wondering if this still works?
> 
> We have:
> 
> #ifdef CONFIG_PREEMPT_COUNT
> #define PREEMPT_DISABLED	(1 + PREEMPT_ENABLED)
> #else
> #define PREEMPT_DISABLED	PREEMPT_ENABLED
> #endif
> 
> Now if we just remove the PREEMPT_ACTIVE from the INIT_PREEMPT_COUNT,
> when CONFIG_PREEMPT_COUNT is not set, wouldn't a _cond_resched() in
> boot up schedule?

Good point, PREEMPT_COUNT isn't required for voluntary preemption; when
I tested the voluntary build nothing bad happened, but that could have
been luck of course.

So what we can do is use an unconditional 1 for INIT_PREEMPT_COUNT, such
that the init task will have a raised preempt_count() during boot, even
on !PREEMPT_COUNT kernels.

But that means we cannot use INIT_PREEMPT_COUNT for
init_task_preempt_count() and such like, as I wanted to. I'll make
FORK_PREEMPT_COUNT for that instead.
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