Re: [PATCH RFC v3 03/13] rcu-tasks: Add a Tasks RCU implementation for reader-marked trampolines

[Frederic Weisbecker]

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