kick_all_cpus_sync() is used to broadcast IPIs to all online CPUs to force them synchronize caches, TLB etc. It is called only 3 times - from mm/slab arm64 and powerpc code. We can delay synchronization work for CPUs in extended quiescent state (idle or nohz_full userspace). As Paul E. McKenney wrote: -- Currently, IPIs are used to force other CPUs to invalidate their TLBs in response to a kernel virtual-memory mapping change. This works, but degrades both battery lifetime (for idle CPUs) and real-time response (for nohz_full CPUs), and in addition results in unnecessary IPIs due to the fact that CPUs executing in usermode are unaffected by stale kernel mappings. It would be better to cause a CPU executing in usermode to wait until it is entering kernel mode to do the flush, first to avoid interrupting usemode tasks and second to handle multiple flush requests with a single flush in the case of a long-running user task. -- v2 is big rework to address comments in v1: - rcu_eqs_special() declaration in public header is dropped, it is not used in new implementation. Though, I hope Paul will pick it in his tree; - for arm64, few isb() added to ensure kernel text synchronization (patches 1-4); - rcu_get_eqs_cpus() introduced and used to mask EQS CPUs before generating broadcast IPIs; - RCU_DYNTICK_CTRL_MASK is not touched because memory barrier is implicitly issued in EQS exit path; - powerpc is not an exception anymore. I think it's safe to delay synchronization for it as well, and I didn't get comments from ppc community. v1: https://lkml.org/lkml/2018/3/25/109 Based on next-20180405 Yury Norov (5): arm64: entry: isb in el1_irq arm64: entry: introduce restore_syscall_args macro arm64: ISB early at exit from extended quiescent state rcu: arm64: add rcu_dynticks_eqs_exit_sync() smp: Lazy synchronization for EQS CPUs in kick_all_cpus_sync() arch/arm64/kernel/Makefile | 2 ++ arch/arm64/kernel/entry.S | 52 +++++++++++++++++++++++++++++++-------------- arch/arm64/kernel/process.c | 7 ++++++ arch/arm64/kernel/rcu.c | 8 +++++++ include/linux/rcutiny.h | 2 ++ include/linux/rcutree.h | 1 + kernel/rcu/tiny.c | 9 ++++++++ kernel/rcu/tree.c | 27 +++++++++++++++++++++++ kernel/smp.c | 21 +++++++++++------- 9 files changed, 105 insertions(+), 24 deletions(-) create mode 100644 arch/arm64/kernel/rcu.c -- 2.14.1
