[RFC PATCH 0/2] Switch tracing from sched-RCU to preempt-RCU

This is a follow-up on
	https://lore.kernel.org/all/20241206120709.736f943e@gandalf.local.home/T/#u (local)

The problem is that trace points disable preemption due to sched-RCU.
The arguments passed to tracepoints may used locks (yes, they should
not) and eBPF programs can be attached to tracepoints which may use
locks which become sleeping locks on PREEMPT_RT.

Mathieu said regarding the need for preempt_notrace:
| There are a few things to consider here about the constraints of the
| callsites where the tracepoints are inserted. In general, those need to
| be:
| 
| - NMI-safe
| - notrace
| - usable from the scheduler (with rq lock held)
| - usable to trace the RCU implementation

This is covered by the here suggested rcu_read_lock_notrace().

Mathieu's suggested steps were:

| Well the first step would be to introduce a rcu_read_lock/unlock_notrace.

Patch #1

| This solves the problem at the tracepoint level, but requires that we
| initially move the preempt disable to the tracer callbacks. Then we
| can figure out within each tracer what needs to be done to further
| reduce the preempt off critical section.

Here I am stuck. Patch #2 is probably not correct due to "move the
preempt disable to the tracer callbacks". I didn't figure out where they
are…

Sebastian Andrzej Siewior (2):
  rcu: Add rcu_read_lock_notrace()
  trace: Use rcu_read_lock() instead preempt_disable()

 include/linux/rcupdate.h    | 41 +++++++++++++++++++++++++++++++++++++
 include/linux/tracepoint.h  |  2 +-
 kernel/rcu/tree_plugin.h    | 14 +++++++++++++
 kernel/trace/trace_events.c |  8 +-------
 4 files changed, 57 insertions(+), 8 deletions(-)

-- 
2.49.0