On 2018-11-08 10:05:17 [-0800], Paul E. McKenney wrote: > Just to make sure I understand, this is the call to queue_delayed_work_on() > from srcu_queue_delayed_work_on(), right? correct. > And if I am guessing correctly, you would like to get rid of the > constraint requiring CPUHP_RCUTREE_PREP to precede CPUHP_TIMERS_PREPARE? no, my problem is the preempt_disable() around queue_delayed_work_on(). If the CPUs goes offline _after_ queue_delayed_work_on() then the timer gets migrated and work item should show up on another CPU. If the CPU is offline at queue_delayed_work_on() time then the timer gets enqueued and won't fire until the CPU is back online and I *think* that is the reason behind this "is CPU online" check. > If so, the swait_event_idle_timeout_exclusive() in rcu_gp_fqs_loop() > in kernel/rcu/tree.c also requires this ordering. There are probably > other pieces of code needing this. > > Plus the reason for running this on a specific CPU is that the workqueue > item is processing that CPU's per-CPU variables, including invoking that > CPU's callbacks. The item is srcu_invoke_callbacks(). The SRCU callback is invoking per-CPU variables? Like this_cpu_ptr()? But if the CPU is offline then you fallback to queue_delayed_work()? > Thanx, Paul Sebastian
