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 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?
> 
> 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.

-- 
Frederic Weisbecker
SUSE Labs