From: Peter Zijlstra <[email protected]> The sync_rcu stuff is specificically for clearing bits in the active mask, such that everybody will observe the bit cleared and will not consider the cleared CPU for load-balancing etc.
Signed-off-by: Peter Zijlstra (Intel) <[email protected]> Signed-off-by: Thomas Gleixner <[email protected]> --- kernel/cpu.c | 15 --------------- kernel/sched/core.c | 14 ++++++++++++++ 2 files changed, 14 insertions(+), 15 deletions(-) --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -691,21 +691,6 @@ static int takedown_cpu(unsigned int cpu struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu); int err; - /* - * By now we've cleared cpu_active_mask, wait for all preempt-disabled - * and RCU users of this state to go away such that all new such users - * will observe it. - * - * For CONFIG_PREEMPT we have preemptible RCU and its sync_rcu() might - * not imply sync_sched(), so wait for both. - * - * Do sync before park smpboot threads to take care the rcu boost case. - */ - if (IS_ENABLED(CONFIG_PREEMPT)) - synchronize_rcu_mult(call_rcu, call_rcu_sched); - else - synchronize_rcu(); - /* Park the hotplug thread */ kthread_park(per_cpu_ptr(&cpuhp_state, cpu)->thread); --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -7345,6 +7345,20 @@ int sched_cpu_deactivate(unsigned int cp int ret; set_cpu_active(cpu, false); + /* + * We've cleared cpu_active_mask, wait for all preempt-disabled and RCU + * users of this state to go away such that all new such users will + * observe it. + * + * For CONFIG_PREEMPT we have preemptible RCU and its sync_rcu() might + * not imply sync_sched(), so wait for both. + * + * Do sync before park smpboot threads to take care the rcu boost case. + */ + if (IS_ENABLED(CONFIG_PREEMPT)) + synchronize_rcu_mult(call_rcu, call_rcu_sched); + else + synchronize_rcu(); if (!sched_smp_initialized) return 0;
