Messages in this thread | | | Date | Fri, 19 May 2017 06:35:50 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Use case for TASKS_RCU |
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On Fri, May 19, 2017 at 08:23:31AM +0200, Ingo Molnar wrote: > > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote: > > > On Tue, May 16, 2017 at 08:22:33AM +0200, Ingo Molnar wrote: > > > > > > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote: > > > > > > > Hello! > > > > > > > > The question of the use case for TASKS_RCU came up, and here is my > > > > understanding. Steve will not be shy about correcting any misconceptions > > > > I might have. ;-) > > > > > > > > The use case is to support freeing of trampolines used in tracing/probing > > > > in CONFIG_PREEMPT=y kernels. It is necessary to wait until any task > > > > executing in the trampoline in question has left it, taking into account > > > > that the trampoline's code might be interrupted and preempted. However, > > > > the code in the trampolines is guaranteed never to context switch. > > > > > > > > Note that in CONFIG_PREEMPT=n kernels, synchronize_sched() suffices. > > > > It is therefore tempting to think in terms of disabling preemption across > > > > the trampolines, but there is apparently not enough room to accommodate > > > > the needed preempt_disable() and preempt_enable() in the code invoking > > > > the trampoline, and putting the preempt_disable() and preempt_enable() > > > > in the trampoline itself fails because of the possibility of preemption > > > > just before the preempt_disable() and just after the preempt_enable(). > > > > Similar reasoning rules out use of rcu_read_lock() and rcu_read_unlock(). > > > > > > So how was this solved before TASKS_RCU? Also, nothing uses call_rcu_tasks() at > > > the moment, so it's hard for me to review its users. What am I missing? > > > > Before TASKS_RCU, the trampolines were just leaked when CONFIG_PREEMPT=y. > > > > Current mainline kernel/trace/ftrace.c uses synchronize_rcu_tasks(). > > So yes, currently one user. > > So why not schedule a worklet on every CPU to drive the trampoline freeing? To > guarantee that nothing was preempted it could run at SCHED_IDLE and could observe > nr_running from the worklet and use a short timeout loop. Batching and hysteresis > would ensure that this is only running rarely in practice. > > It doesn't have to be fast or particularly elegant, but it could use existing > kernel facilites just fine: it's a corner case cost and quirk of our live kernel > text modifying trampoline code and our current CONFIG_PREEMPT=y model. > > I.e. don't make it an RCU facility that complicates not just the RCU code but has > various costs in generic code as well: > > kernel/exit.c: TASKS_RCU(int tasks_rcu_i); > kernel/exit.c: TASKS_RCU(preempt_disable()); > kernel/exit.c: TASKS_RCU(tasks_rcu_i = __srcu_read_lock(&tasks_rcu_exit_srcu)); > kernel/exit.c: TASKS_RCU(preempt_enable()); > kernel/exit.c: TASKS_RCU(__srcu_read_unlock(&tasks_rcu_exit_srcu, tasks_rcu_i)); > > I.e. I question that this should be a generic RCU facility.
Simpler would be better!
However, is it really guaranteed that one SCHED_IDLE thread cannot preempt another? If not, then the trampoline-freeing SCHED_IDLE thread might preempt some other SCHED_IDLE thread in the middle of a trampoline. I am not seeing anything that prevents such preemption, but it is rather early local time, so I could easily be missing something.
However, if SCHED_IDLE threads cannot preempt other threads, even other SCHED_IDLE threads, then your approach sounds quite promising to me.
Steve, Peter, thoughts?
Thanx, Paul
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