On Thu, Feb 29, 2024 at 12:41:55PM -0500, Joel Fernandes wrote: > > On Feb 29, 2024, at 11:57 AM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > On Thu, Feb 29, 2024 at 09:21:48AM -0500, Joel Fernandes wrote: > >>> On 2/28/2024 5:58 PM, Paul E. McKenney wrote: > >>> On Wed, Feb 28, 2024 at 02:48:44PM -0800, Alexei Starovoitov wrote: > >>>> On Wed, Feb 28, 2024 at 2:31 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > >>>>> > >>>>> On Wed, 28 Feb 2024 14:19:11 -0800 > >>>>> "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote: > >>>>> > >>>>>>>> > >>>>>>>> Well, to your initial point, cond_resched() does eventually invoke > >>>>>>>> preempt_schedule_common(), so you are quite correct that as far as > >>>>>>>> Tasks RCU is concerned, cond_resched() is not a quiescent state. > >>>>>>> > >>>>>>> Thanks for confirming. :-) > >>>>>> > >>>>>> However, given that the current Tasks RCU use cases wait for trampolines > >>>>>> to be evacuated, Tasks RCU could make the choice that cond_resched() > >>>>>> be a quiescent state, for example, by adjusting rcu_all_qs() and > >>>>>> .rcu_urgent_qs accordingly. > >>>>>> > >>>>>> But this seems less pressing given the chance that cond_resched() might > >>>>>> go away in favor of lazy preemption. > >>>>> > >>>>> Although cond_resched() is technically a "preemption point" and not truly a > >>>>> voluntary schedule, I would be happy to state that it's not allowed to be > >>>>> called from trampolines, or their callbacks. Now the question is, does BPF > >>>>> programs ever call cond_resched()? I don't think they do. > >>>>> > >>>>> [ Added Alexei ] > >>>> > >>>> I'm a bit lost in this thread :) > >>>> Just answering the above question. > >>>> bpf progs never call cond_resched() directly. > >>>> But there are sleepable (aka faultable) bpf progs that > >>>> can call some helper or kfunc that may call cond_resched() > >>>> in some path. > >>>> sleepable bpf progs are protected by rcu_tasks_trace. > >>>> That's a very different one vs rcu_tasks. > >>> > >>> Suppose that the various cond_resched() invocations scattered throughout > >>> the kernel acted as RCU Tasks quiescent states, so that as soon as a > >>> given task executed a cond_resched(), synchronize_rcu_tasks() might > >>> return or call_rcu_tasks() might invoke its callback. > >>> > >>> Would that cause BPF any trouble? > >>> > >>> My guess is "no", because it looks like BPF is using RCU Tasks (as you > >>> say, as opposed to RCU Tasks Trace) only to wait for execution to leave a > >>> trampoline. But I trust you much more than I trust myself on this topic! > >> > >> But it uses RCU Tasks Trace as well (for sleepable bpf programs), not just > >> Tasks? Looks like that's what Alexei said above as well, and I confirmed it in > >> bpf/trampoline.c > >> > >> /* The trampoline without fexit and fmod_ret progs doesn't call original > >> * function and doesn't use percpu_ref. > >> * Use call_rcu_tasks_trace() to wait for sleepable progs to finish. > >> * Then use call_rcu_tasks() to wait for the rest of trampoline asm > >> * and normal progs. > >> */ > >> call_rcu_tasks_trace(&im->rcu, __bpf_tramp_image_put_rcu_tasks); > >> > >> The code comment says it uses both. > > > > BPF does quite a few interesting things with these. > > > > But would you like to look at the update-side uses of RCU Tasks Rude > > to see if lazy preemption affects them? I don't believe that there > > are any problems here, but we do need to check. > > Sure I will be happy to. I am planning look at it in detail over the 3 day weekend. Too much fun! ;-) Thank you, and looking forward to seeing what you come up with! The canonical concern would be that someone somewhere is using either call_rcu_tasks_rude() or synchronize_rcu_tasks_rude() to wait for non-preemptible regions of code that does not account for the possibility of preemption in CONFIG_PREEMPT_NONE or PREEMPT_PREEMPT_VOLUNTARY kernels. I *think* that these are used only to handle the possibility of tracepoints on functions on the entry/exit path and on the RCU-not-watching portions of the idle loop. If so, then there is no difference in behavior for lazy preemption. But who knows? Thanx, Paul
