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Subject[tip:sched/urgent] sched: Remove PREEMPT_NEED_RESCHED from generic code
Commit-ID:  ba1f14fbe70965ae0fb1655a5275a62723f65b77
Gitweb: http://git.kernel.org/tip/ba1f14fbe70965ae0fb1655a5275a62723f65b77
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Thu, 28 Nov 2013 14:26:41 +0100
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 11 Dec 2013 15:52:32 +0100

sched: Remove PREEMPT_NEED_RESCHED from generic code

While hunting a preemption issue with Alexander, Ben noticed that the
currently generic PREEMPT_NEED_RESCHED stuff is horribly broken for
load-store architectures.

We currently rely on the IPI to fold TIF_NEED_RESCHED into
PREEMPT_NEED_RESCHED, but when this IPI lands while we already have
a load for the preempt-count but before the store, the store will erase
the PREEMPT_NEED_RESCHED change.

The current preempt-count only works on load-store archs because
interrupts are assumed to be completely balanced wrt their preempt_count
fiddling; the previous preempt_count load will match the preempt_count
state after the interrupt and therefore nothing gets lost.

This patch removes the PREEMPT_NEED_RESCHED usage from generic code and
pushes it into x86 arch code; the generic code goes back to relying on
TIF_NEED_RESCHED.

Boot tested on x86_64 and compile tested on ppc64.

Reported-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reported-and-Tested-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20131128132641.GP10022@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/include/asm/preempt.h | 11 +++++++++++
include/asm-generic/preempt.h | 35 +++++++++++------------------------
include/linux/sched.h | 2 --
3 files changed, 22 insertions(+), 26 deletions(-)

diff --git a/arch/x86/include/asm/preempt.h b/arch/x86/include/asm/preempt.h
index 8729723..c8b0519 100644
--- a/arch/x86/include/asm/preempt.h
+++ b/arch/x86/include/asm/preempt.h
@@ -8,6 +8,12 @@
DECLARE_PER_CPU(int, __preempt_count);

/*
+ * We use the PREEMPT_NEED_RESCHED bit as an inverted NEED_RESCHED such
+ * that a decrement hitting 0 means we can and should reschedule.
+ */
+#define PREEMPT_ENABLED (0 + PREEMPT_NEED_RESCHED)
+
+/*
* We mask the PREEMPT_NEED_RESCHED bit so as not to confuse all current users
* that think a non-zero value indicates we cannot preempt.
*/
@@ -74,6 +80,11 @@ static __always_inline void __preempt_count_sub(int val)
__this_cpu_add_4(__preempt_count, -val);
}

+/*
+ * Because we keep PREEMPT_NEED_RESCHED set when we do _not_ need to reschedule
+ * a decrement which hits zero means we have no preempt_count and should
+ * reschedule.
+ */
static __always_inline bool __preempt_count_dec_and_test(void)
{
GEN_UNARY_RMWcc("decl", __preempt_count, __percpu_arg(0), "e");
diff --git a/include/asm-generic/preempt.h b/include/asm-generic/preempt.h
index ddf2b42..1cd3f5d 100644
--- a/include/asm-generic/preempt.h
+++ b/include/asm-generic/preempt.h
@@ -3,13 +3,11 @@

#include <linux/thread_info.h>

-/*
- * We mask the PREEMPT_NEED_RESCHED bit so as not to confuse all current users
- * that think a non-zero value indicates we cannot preempt.
- */
+#define PREEMPT_ENABLED (0)
+
static __always_inline int preempt_count(void)
{
- return current_thread_info()->preempt_count & ~PREEMPT_NEED_RESCHED;
+ return current_thread_info()->preempt_count;
}

static __always_inline int *preempt_count_ptr(void)
@@ -17,11 +15,6 @@ static __always_inline int *preempt_count_ptr(void)
return &current_thread_info()->preempt_count;
}

-/*
- * We now loose PREEMPT_NEED_RESCHED and cause an extra reschedule; however the
- * alternative is loosing a reschedule. Better schedule too often -- also this
- * should be a very rare operation.
- */
static __always_inline void preempt_count_set(int pc)
{
*preempt_count_ptr() = pc;
@@ -41,28 +34,17 @@ static __always_inline void preempt_count_set(int pc)
task_thread_info(p)->preempt_count = PREEMPT_ENABLED; \
} while (0)

-/*
- * We fold the NEED_RESCHED bit into the preempt count such that
- * preempt_enable() can decrement and test for needing to reschedule with a
- * single instruction.
- *
- * We invert the actual bit, so that when the decrement hits 0 we know we both
- * need to resched (the bit is cleared) and can resched (no preempt count).
- */
-
static __always_inline void set_preempt_need_resched(void)
{
- *preempt_count_ptr() &= ~PREEMPT_NEED_RESCHED;
}

static __always_inline void clear_preempt_need_resched(void)
{
- *preempt_count_ptr() |= PREEMPT_NEED_RESCHED;
}

static __always_inline bool test_preempt_need_resched(void)
{
- return !(*preempt_count_ptr() & PREEMPT_NEED_RESCHED);
+ return false;
}

/*
@@ -81,7 +63,12 @@ static __always_inline void __preempt_count_sub(int val)

static __always_inline bool __preempt_count_dec_and_test(void)
{
- return !--*preempt_count_ptr();
+ /*
+ * Because of load-store architectures cannot do per-cpu atomic
+ * operations; we cannot use PREEMPT_NEED_RESCHED because it might get
+ * lost.
+ */
+ return !--*preempt_count_ptr() && tif_need_resched();
}

/*
@@ -89,7 +76,7 @@ static __always_inline bool __preempt_count_dec_and_test(void)
*/
static __always_inline bool should_resched(void)
{
- return unlikely(!*preempt_count_ptr());
+ return unlikely(!preempt_count() && tif_need_resched());
}

#ifdef CONFIG_PREEMPT
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 768b037..96d674b 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -440,8 +440,6 @@ struct task_cputime {
.sum_exec_runtime = 0, \
}

-#define PREEMPT_ENABLED (PREEMPT_NEED_RESCHED)
-
#ifdef CONFIG_PREEMPT_COUNT
#define PREEMPT_DISABLED (1 + PREEMPT_ENABLED)
#else

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