On Wed, Jul 30, 2014 at 07:46:30PM +0200, Oleg Nesterov wrote: > On 07/30, Frederic Weisbecker wrote: > > > > On Tue, Jul 29, 2014 at 07:54:14PM +0200, Oleg Nesterov wrote: > > > > > > > > Looks like, we can kill context_tracking_task_switch() and simply change the > > > "__init" callers of context_tracking_cpu_set() to do set_thread_flag(TIF_NOHZ) ? > > > Then this flag will be propagated by copy_process(). > > > > Right, that would be much better. Good catch! context tracking is enabled from > > tick_nohz_init(). This is the init 0 task so the flag should be propagated from there. > > actually init 1 task, but this doesn't matter. Are you sure? It does matter because that would invalidate everything I understood about init/main.c :) I was convinced that the very first kernel init task is PID 0 then it forks on rest_init() to launch the userspace init with PID 1. Then init/0 becomes the idle task of the boot CPU. > > > I still think we need a for_each_process_thread() set as well though because some > > kernel threads may well have been created at this stage already. > > Yes... Or we can add set_thread_flag(TIF_NOHZ) into ____call_usermodehelper(). Couldn't there be some other tasks than usermodehelper stuffs at this stage? Like workqueues or random kernel threads? > > > > Or I am totally confused? (quite possible). > > > > > > > So here is a scenario where this is a problem: a task runs on CPU 0, passes the context > > > > tracking call before returning from a syscall to userspace, and gets an interrupt. The > > > > interrupt preempts the task and it moves to CPU 1. So it returns from preempt_schedule_irq() > > > > after which it is going to resume to userspace. > > > > > > > > In this scenario, if context tracking is only enabled on CPU 1, we have no way to know that > > > > the task is resuming to userspace, because we passed through the context tracking probe > > > > already and it was ignored on CPU 0. > > > > > > Thanks. But I still can't understand... So if we only track CPU 1, then in this > > > case context_tracking.state == IN_USER on CPU 0, but it can be IN_USER or IN_KERNEL > > > on CPU 1. > > > > I'm not sure I understand your question. > > Probably because it was stupid. Seriously, I still have no idea what this code > actually does. > > > Context tracking is either enabled everywhere or > > nowhere. > > > > I need to say though that there is a per CPU context tracking state named context_tracking.active. > > It's confusing because it suggests that context tracking is active per CPU. Actually it's tracked > > everywhere when globally enabled, but active determines if we call the RCU and vtime callbacks or > > not. > > > > So only nohz full CPUs have context_tracking.active set because only these need to call the RCU > > and vtime callbacks. Other CPUs still do the context tracking but they won't call rcu and vtime > > functions. > > I meant that in the scenario you described above the "global" TIF_NOHZ doesn't > really make a difference, afaics. > > Lets assume that context tracking is only enabled on CPU 1. To simplify, > assume that we have a single usermode task T which sleeps in kernel mode. > > So context_tracking[0].state == context_tracking[1].state == IN_KERNEL. > > T wakes up on CPU_0, returns to user space, calls user_enter(). This sets > context_tracking[0].state = IN_USER but otherwise does nothing else, this > CPU is not tracked and .active is false. > > Right after local_irq_restore() this task can migrate to CPU_1 and finish > its ret-to-usermode path. But since it had already passed user_enter() we > do not change context_tracking[1].state and do not play with rcu/vtime. > (unless this task hits SCHEDULE_USER in asm). > > The same for user_exit() of course. So indeed if context tracking is enabled on CPU 1 and not in CPU 0, we risk such situation where CPU 1 has wrong context tracking. But global TIF_NOHZ should enforce context tracking everywhere. And also it's less context switch overhead. > > Oleg. >
