[PATCH] sched: Revert need_resched() to look at TIF_NEED_RESCHED

[Peter Zijlstra]

Subject: sched: Revert need_resched() to look at TIF_NEED_RESCHED
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
 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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