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[RFC][PATCH 09/11] sched, x86: Kill saved_preempt_count

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  [RFC][PATCH 09/11] sched, x86: Kill saved_preempt_count Peter Zijlstra <peterz@infradead.org> - 2015-09-29 11:50 +0200
    Re: [RFC][PATCH 09/11] sched, x86: Kill saved_preempt_count Steven Rostedt <rostedt@goodmis.org> - 2015-09-29 17:50 +0200
      Re: [RFC][PATCH 09/11] sched, x86: Kill saved_preempt_count Peter Zijlstra <peterz@infradead.org> - 2015-09-29 18:00 +0200

#1234927 — [RFC][PATCH 09/11] sched, x86: Kill saved_preempt_count

With the introduction of the context switch preempt_count invariant,
and the demise of PREEMPT_ACTIVE, its pointless to save/restore the
per-cpu preemption count, it must always be 2.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/include/asm/preempt.h     |    6 +-----
 arch/x86/include/asm/thread_info.h |    2 --
 arch/x86/kernel/process_32.c       |    8 --------
 arch/x86/kernel/process_64.c       |    8 --------
 4 files changed, 1 insertion(+), 23 deletions(-)

--- a/arch/x86/include/asm/preempt.h
+++ b/arch/x86/include/asm/preempt.h
@@ -30,13 +30,9 @@ 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 = \
-		2*PREEMPT_DISABLE_OFFSET + PREEMPT_NEED_RESCHED; \
-} while (0)
+#define init_task_preempt_count(p) do { } while (0)
 
 #define init_idle_preempt_count(p, cpu) do { \
-	task_thread_info(p)->saved_preempt_count = PREEMPT_ENABLED; \
 	per_cpu(__preempt_count, (cpu)) = PREEMPT_ENABLED; \
 } while (0)
 
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -57,7 +57,6 @@ struct thread_info {
 	__u32			flags;		/* low level flags */
 	__u32			status;		/* thread synchronous flags */
 	__u32			cpu;		/* current CPU */
-	int			saved_preempt_count;
 	mm_segment_t		addr_limit;
 	void __user		*sysenter_return;
 	unsigned int		sig_on_uaccess_error:1;
@@ -69,7 +68,6 @@ struct thread_info {
 	.task		= &tsk,			\
 	.flags		= 0,			\
 	.cpu		= 0,			\
-	.saved_preempt_count = INIT_PREEMPT_COUNT,	\
 	.addr_limit	= KERNEL_DS,		\
 }
 
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -280,14 +280,6 @@ __switch_to(struct task_struct *prev_p,
 		set_iopl_mask(next->iopl);
 
 	/*
-	 * If it were not for PREEMPT_ACTIVE we could guarantee that the
-	 * preempt_count of all tasks was equal here and this would not be
-	 * needed.
-	 */
-	task_thread_info(prev_p)->saved_preempt_count = this_cpu_read(__preempt_count);
-	this_cpu_write(__preempt_count, task_thread_info(next_p)->saved_preempt_count);
-
-	/*
 	 * Now maybe handle debug registers and/or IO bitmaps
 	 */
 	if (unlikely(task_thread_info(prev_p)->flags & _TIF_WORK_CTXSW_PREV ||
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -401,14 +401,6 @@ __switch_to(struct task_struct *prev_p,
 	 */
 	this_cpu_write(current_task, next_p);
 
-	/*
-	 * If it were not for PREEMPT_ACTIVE we could guarantee that the
-	 * preempt_count of all tasks was equal here and this would not be
-	 * needed.
-	 */
-	task_thread_info(prev_p)->saved_preempt_count = this_cpu_read(__preempt_count);
-	this_cpu_write(__preempt_count, task_thread_info(next_p)->saved_preempt_count);
-
 	/* Reload esp0 and ss1.  This changes current_thread_info(). */
 	load_sp0(tss, next);
 


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#1235316

On Tue, 29 Sep 2015 11:28:34 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> With the introduction of the context switch preempt_count invariant,
> and the demise of PREEMPT_ACTIVE, its pointless to save/restore the
> per-cpu preemption count, it must always be 2.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Should we add a warn on someplace:

 WARN_ON_ONCE(preempt_count() != 2)

?

-- Steve
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#1235338

On Tue, Sep 29, 2015 at 11:44:04AM -0400, Steven Rostedt wrote:
> On Tue, 29 Sep 2015 11:28:34 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > With the introduction of the context switch preempt_count invariant,
> > and the demise of PREEMPT_ACTIVE, its pointless to save/restore the
> > per-cpu preemption count, it must always be 2.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> 
> Should we add a warn on someplace:
> 
>  WARN_ON_ONCE(preempt_count() != 2)

See v2 12/11 :-)
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