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authorMartin Schwidefsky <schwidefsky@de.ibm.com>2019-05-17 12:50:42 +0200
committerHeiko Carstens <heiko.carstens@de.ibm.com>2019-06-15 12:25:52 +0200
commit38f2c691a4b3e89d476f8e8350d1ca299974b89d (patch)
tree8b4ab76fd8281be6dfcf8d5edc48d5ee522a4710 /arch/s390/kernel/smp.c
parent7928260539f3a13b5b23a3fa0a7c0e4f5255940b (diff)
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s390: improve wait logic of stop_machine
The stop_machine loop to advance the state machine and to wait for all affected CPUs to check-in calls cpu_relax_yield in a tight loop until the last missing CPUs acknowledged the state transition. On a virtual system where not all logical CPUs are backed by real CPUs all the time it can take a while for all CPUs to check-in. With the current definition of cpu_relax_yield a diagnose 0x44 is done which tells the hypervisor to schedule *some* other CPU. That can be any CPU and not necessarily one of the CPUs that need to run in order to advance the state machine. This can lead to a pretty bad diagnose 0x44 storm until the last missing CPU finally checked-in. Replace the undirected cpu_relax_yield based on diagnose 0x44 with a directed yield. Each CPU in the wait loop will pick up the next CPU in the cpumask of stop_machine. The diagnose 0x9c is used to tell the hypervisor to run this next CPU instead of the current one. If there is only a limited number of real CPUs backing the virtual CPUs we end up with the real CPUs passed around in a round-robin fashion. [heiko.carstens@de.ibm.com]: Use cpumask_next_wrap as suggested by Peter Zijlstra. Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Diffstat (limited to 'arch/s390/kernel/smp.c')
-rw-r--r--arch/s390/kernel/smp.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c
index f00955940694..44974654cbd0 100644
--- a/arch/s390/kernel/smp.c
+++ b/arch/s390/kernel/smp.c
@@ -414,7 +414,7 @@ void smp_yield_cpu(int cpu)
diag_stat_inc_norecursion(DIAG_STAT_X09C);
asm volatile("diag %0,0,0x9c"
: : "d" (pcpu_devices[cpu].address));
- } else if (MACHINE_HAS_DIAG44) {
+ } else if (MACHINE_HAS_DIAG44 && !smp_cpu_mtid) {
diag_stat_inc_norecursion(DIAG_STAT_X044);
asm volatile("diag 0,0,0x44");
}