Le Wed, Sep 16, 2026 at 07:55:02AM -0700, Paul E. McKenney a écrit : > On Wed, Sep 16, 2026 at 04:47:29PM +0200, Frederic Weisbecker wrote: > > Le Wed, Sep 16, 2026 at 04:35:50PM +0200, Frederic Weisbecker a écrit : > > > Le Wed, Sep 16, 2026 at 07:26:35AM -0700, Paul E. McKenney a écrit : > > > > On Wed, Sep 16, 2026 at 02:40:20PM +0200, Frederic Weisbecker wrote: > > > > > Le Tue, Sep 15, 2026 at 04:56:41PM -0700, Paul E. McKenney a écrit : > > > > > > > Alternatively the approach could be generalized to vanilla RCU, > > > > > > > it could be > > > > > > > possible to define a .text.rcu_no_qs section within which code > > > > > > > running is > > > > > > > considered as an RCU reader (with a pause while on the explicit > > > > > > > RCU tasks > > > > > > > section). It would be forbidden to voluntary sleep inside > > > > > > > and to put explicit preemption points (CONFIG_PROVE_RCU could > > > > > > > report misuses). > > > > > > > > > > > > > > Based on IP, RCU could consider those interrupted section as > > > > > > > readers. This would > > > > > > > require PREEMPT_RCU though. > > > > > > > > > > > > > > And then synchronize_rcu() would do the 1, 2, 4 jobs. > > > > > > > > > > > > If I am following correctly (ha!), sleepable BPF programs rule out > > > > > > use > > > > > > of RCU in this manner. > > > > > > > > > > > > But your point is nevertheless valid, in that SRCU could be used. > > > > > > And because rcu_read_lock_trace() is a thin wrapper around > > > > > > SRCU-fast, we > > > > > > *might* be able to instead use rcu_read_lock_tasks_trace(), which > > > > > > would > > > > > > skip the task-struct increment and decrement, saving a few > > > > > > instructions. > > > > > > Then, instead of waiting for each task's counter to go to zero, > > > > > > instead > > > > > > just invoke synchronize_rcu_tasks_trace(). > > > > > > > > > > > > Which is pretty close to what Josef is proposing, just with the new > > > > > > RCU > > > > > > Tasks Trace read-side primitives. I think. ;-) > > > > > > > > > > > > This assumes that we do not need to flatten partially overlapping > > > > > > RCU > > > > > > Tasks Trace readers into one big reader. > > > > > > > > > > > > Or am I missing something here? > > > > > > > > > > Yes I think that's what Josef does in this patchset. The problem is > > > > > about > > > > > handling the few instructions: > > > > > > > > > > 1) between the begining of the trampoline and the call to > > > > > rcu_read_lock_trace() > > > > > > > > > > 2) between the call to rcu_read_unlock_trace() and the end of the > > > > > trampoline > > > > > > > > > > So what I'm proposing is to make those two parts implicit RCU read > > > > > lock sections. > > > > > > > > > > So the whole trampoline would be .text.rcu_no_qs: > > > > > > > > > > .text.rcu_no_qs trampoline: > > > > > > > > > > __________________________________________________________________________________________ > > > > > |Few instructions 1 | rcu_read_lock_trace() .... > > > > > rcu_read_unlock_trace | Few instructions 2| > > > > > > > > > > ___________________________________________________________________________________________ > > > > > > > > > > Then when a tick fires, rcu_flavor_sched_clock_irq() discards the > > > > > interrupted > > > > > code as QS if the IP was within .text.rcu_no_qs _unless_ it is in the > > > > > rcu_read_lock_trace. Both are easy and quick to verify. > > > > > > > > > > Also preempt_schedule_irq() would make sure to verify the same > > > > > condition and > > > > > enqueue the task as a GP blocker if preempting inside "Few > > > > > instructions 1" > > > > > or "Few instructions 2". > > > > > > > > Ah, OK, I might be following now. ;-) > > > > > > > > We also need both versions of rcu_exp_handler() to check the IP as well, > > > > given that sooner or later someone is going to want trampoline removal > > > > to go faster. Or am I still missing a turn in here somewhere? > > I should add that the thing that I really like about Frederic's approach > is that avoids the task-list scan. Or at least has the potential to > do so. Such scans have proven problematic in the past. > > > > Yes indeed, missed the exp part! > > > > What remains to handle also is non-preemptible RCU because if the task is > > preempted by an IRQ while in the .text.rcu_no_qs, we may still need to keep > > track of that somewhere. > > Perhaps in rcu_core() in kernels booted with use_softirq? I am thinking > specifically of the checks for deferred quiescent states. I don't (yet) > see a need to modify rcu_check_quiescent_state(). > > Maybe other places as well. ;-)
Hmm this tracking would have to happen on preempt_schedule() just like we do for PREEMPT_RCU. Or am I missing something? And then we would need a list scan of those tasks. Or we can build the blocked task list handling, that we already have for PREEMPT_RCU, when CONFIG_RCU_TASKS && !CONFIG_PREEMPT_RCU. We would just only add tasks when preempted in .text.rcu_no_qs since rcu_read_lock() would still disable preemption on normal explicit readers. So I wouldn't expect more overhead due to that blocked list tracking built since it would rarely track tasks. Thanks. -- Frederic Weisbecker SUSE Labs
