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Subject[PATCH] sched: Revert need_resched() to look at TIF_NEED_RESCHED
Subject: sched: Revert need_resched() to look at TIF_NEED_RESCHED
From: Peter Zijlstra <peterz@infradead.org>
Date: Fri Sep 27 17:20:30 CEST 2013

Yuanhan reported a serious throughput regression in his pigz
benchmark. Using the ftrace patch I found that several idle paths
need more TLC before we can switch the generic need_resched() over to
preempt_need_resched.

The preemption paths benefit most from preempt_need_resched and do
indeed use it; all other need_resched() users don't really care that
much so reverting need_resched() back to tif_need_resched() is the
simple and safe solution.

Reported-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
---
arch/x86/include/asm/preempt.h | 8 --------
include/asm-generic/preempt.h | 8 --------
include/linux/sched.h | 5 +++++
3 files changed, 5 insertions(+), 16 deletions(-)

--- a/arch/x86/include/asm/preempt.h
+++ b/arch/x86/include/asm/preempt.h
@@ -80,14 +80,6 @@ static __always_inline bool __preempt_co
}

/*
- * Returns true when we need to resched -- even if we can not.
- */
-static __always_inline bool need_resched(void)
-{
- return unlikely(test_preempt_need_resched());
-}
-
-/*
* Returns true when we need to resched and can (barring IRQ state).
*/
static __always_inline bool should_resched(void)
--- a/include/asm-generic/preempt.h
+++ b/include/asm-generic/preempt.h
@@ -85,14 +85,6 @@ static __always_inline bool __preempt_co
}

/*
- * Returns true when we need to resched -- even if we can not.
- */
-static __always_inline bool need_resched(void)
-{
- return unlikely(test_preempt_need_resched());
-}
-
-/*
* Returns true when we need to resched and can (barring IRQ state).
*/
static __always_inline bool should_resched(void)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2577,6 +2577,11 @@ static inline bool __must_check current_
}
#endif

+static __always_inline bool need_resched(void)
+{
+ return unlikely(tif_need_resched());
+}
+
/*
* Thread group CPU time accounting.
*/

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