From 4d92f50249eb3ed1c066276e214e8cc7be81e96d Mon Sep 17 00:00:00 2001 From: Heiko Carstens Date: Wed, 28 Jan 2015 07:43:56 +0100 Subject: [PATCH] s390: reintroduce diag 44 calls for cpu_relax() Christian Borntraeger reported that the now missing diag 44 calls (voluntary time slice end) does cause a performance regression for stop_machine() calls if a machine has more virtual cpus than the host has physical cpus. This patch mainly reverts 57f2ffe14fd125c2 ("s390: remove diag 44 calls from cpu_relax()") with the exception that we still do not issue diag 44 calls if running with smt enabled. Due to group scheduling algorithms when running in LPAR this would lead to significant latencies. However, when running in LPAR we do not have more virtual than physical cpus. Reported-and-tested-by: Christian Borntraeger Signed-off-by: Heiko Carstens Signed-off-by: Martin Schwidefsky --- arch/s390/include/asm/processor.h | 5 +---- arch/s390/kernel/processor.c | 10 +++++++++- 2 files changed, 10 insertions(+), 5 deletions(-) diff --git a/arch/s390/include/asm/processor.h b/arch/s390/include/asm/processor.h index bed05ea7ec27..e7cbbdcdee13 100644 --- a/arch/s390/include/asm/processor.h +++ b/arch/s390/include/asm/processor.h @@ -215,10 +215,7 @@ static inline unsigned short stap(void) /* * Give up the time slice of the virtual PU. */ -static inline void cpu_relax(void) -{ - barrier(); -} +void cpu_relax(void); #define cpu_relax_lowlatency() barrier() diff --git a/arch/s390/kernel/processor.c b/arch/s390/kernel/processor.c index dbdd33ee0102..26108232fcaa 100644 --- a/arch/s390/kernel/processor.c +++ b/arch/s390/kernel/processor.c @@ -8,16 +8,24 @@ #include #include -#include #include #include #include #include #include #include +#include static DEFINE_PER_CPU(struct cpuid, cpu_id); +void cpu_relax(void) +{ + if (!smp_cpu_mtid && MACHINE_HAS_DIAG44) + asm volatile("diag 0,0,0x44"); + barrier(); +} +EXPORT_SYMBOL(cpu_relax); + /* * cpu_init - initializes state that is per-CPU. */