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 <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

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()
 

arch/s390/kernel/processor.c

@@ -8,16 +8,24 @@
 
 #include <linux/kernel.h>
 #include <linux/init.h>
-#include <linux/smp.h>
 #include <linux/seq_file.h>
 #include <linux/delay.h>
 #include <linux/cpu.h>
 #include <asm/elf.h>
 #include <asm/lowcore.h>
 #include <asm/param.h>
+#include <asm/smp.h>
 
 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.
  */