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. Thanx, Paul
