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 on s390 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 an architecture specific 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. Patches 1 and 3 are just possible cleanups; the interesting part is patch 2. Heiko Carstens (2): processor: remove spin_cpu_yield processor: get rid of cpu_relax_yield Martin Schwidefsky (1): s390: improve wait logic of stop_machine arch/powerpc/include/asm/processor.h | 2 -- arch/s390/include/asm/processor.h | 7 +------ arch/s390/kernel/processor.c | 21 +++++++++++++++------ arch/s390/kernel/smp.c | 2 +- include/linux/processor.h | 9 --------- include/linux/sched.h | 4 ---- include/linux/stop_machine.h | 1 + kernel/stop_machine.c | 19 ++++++++++++++----- 8 files changed, 32 insertions(+), 33 deletions(-) -- 2.17.1
