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Subject[RFC][PATCH 4/4] arm64: do not use dummy vcpu_is_preempted() anymore
Date
vcpu_is_preempted() now can represent the actual state of
the VCPU, so the scheduler can make better decisions when
it picks the idle CPU to enqueue a task on. I executed a
whole bunch of scheduler tests [0]. One particular test
that shows the importance of vcpu_is_preempted() is AIO
stress-ng test:

x Disabled vcpu_is_preempted()
stress-ng: info: [100] stressor bogo ops real time usr time sys time bogo ops/s bogo ops/s
stress-ng: info: [100] (secs) (secs) (secs) (real time) (usr+sys time)
stress-ng: info: [100] aio 222927 10.01 0.89 27.61 22262.04 7822.00
stress-ng: info: [139] aio 217043 10.01 1.00 26.80 21685.46 7807.30
stress-ng: info: [178] aio 217261 10.01 1.08 26.79 21709.36 7795.51

+ Enabled vcpu_is_preempted()
stress-ng: info: [100] aio 432750 10.00 1.14 19.03 43264.33 21455.13
stress-ng: info: [139] aio 426771 10.01 1.09 18.67 42629.13 21597.72
stress-ng: info: [179] aio 533039 10.00 1.42 20.39 53281.70 24440.12

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
arch/arm64/include/asm/spinlock.h | 17 ++++++++++-------
1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/include/asm/spinlock.h b/arch/arm64/include/asm/spinlock.h
index 9083d6992603..6a390eeabe82 100644
--- a/arch/arm64/include/asm/spinlock.h
+++ b/arch/arm64/include/asm/spinlock.h
@@ -11,17 +11,20 @@
/* See include/linux/spinlock.h */
#define smp_mb__after_spinlock() smp_mb()

-/*
- * Changing this will break osq_lock() thanks to the call inside
- * smp_cond_load_relaxed().
- *
- * See:
- * https://lore.kernel.org/lkml/20200110100612.GC2827@hirez.programming.kicks-ass.net
- */
#define vcpu_is_preempted vcpu_is_preempted
+
+#ifdef CONFIG_PARAVIRT
+extern bool paravirt_vcpu_is_preempted(int cpu);
+
+static inline bool vcpu_is_preempted(int cpu)
+{
+ return paravirt_vcpu_is_preempted(cpu);
+}
+#else
static inline bool vcpu_is_preempted(int cpu)
{
return false;
}
+#endif /* CONFIG_PARAVIRT */

#endif /* __ASM_SPINLOCK_H */
--
2.27.0
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