From 38f2c691a4b3e89d476f8e8350d1ca299974b89d Mon Sep 17 00:00:00 2001 From: Martin Schwidefsky Date: Fri, 17 May 2019 12:50:42 +0200 Subject: 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 Acked-by: Peter Zijlstra (Intel) Acked-by: Thomas Gleixner Signed-off-by: Heiko Carstens --- include/linux/sched.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'include/linux') diff --git a/include/linux/sched.h b/include/linux/sched.h index 11837410690f..1f9f3160da7e 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1519,7 +1519,7 @@ static inline int set_cpus_allowed_ptr(struct task_struct *p, const struct cpuma #endif #ifndef cpu_relax_yield -#define cpu_relax_yield() cpu_relax() +#define cpu_relax_yield(cpumask) cpu_relax() #endif extern int yield_to(struct task_struct *p, bool preempt); -- cgit v1.2.3