Re: RCU-Task[-Trace] VS EQS (was Re: [PATCH v3 13/25] context_tracking, rcu: Rename rcu_dynticks_task*() into rcu_task*())

[Frederic Weisbecker <frederic@kernel.org>]

Le Tue, Jul 30, 2024 at 03:39:44PM -0700, Paul E. McKenney a écrit :
> On Wed, Jul 31, 2024 at 12:17:49AM +0200, Frederic Weisbecker wrote:
> > Le Tue, Jul 30, 2024 at 07:23:58AM -0700, Paul E. McKenney a écrit :
> > > On Thu, Jul 25, 2024 at 04:32:46PM +0200, Frederic Weisbecker wrote:
> > > > Le Wed, Jul 24, 2024 at 04:43:13PM +0200, Valentin Schneider a écrit :
> > > > > -/* Turn on heavyweight RCU tasks trace readers on idle/user entry. */
> > > > > -static __always_inline void rcu_dynticks_task_trace_enter(void)
> > > > > +/* Turn on heavyweight RCU tasks trace readers on kernel exit. */
> > > > > +static __always_inline void rcu_task_trace_exit(void)
> > > > 
> > > > Before I proceed on this last one, a few questions for Paul and others:
> > > > 
> > > > 1) Why is rcu_dynticks_task_exit() not called while entering in NMI?
> > > >    Does that mean that NMIs aren't RCU-Task read side critical sections?
> > > 
> > > Because Tasks RCU Rude handles that case currently.  So good catch,
> > > because this might need adjustment when we get rid of Tasks RCU Rude.
> > > And both rcu_dynticks_task_enter() and rcu_dynticks_task_exit() look safe
> > > to invoke from NMI handlers.  Memory ordering needs checking, of course.
> > > 
> > > Except that on architectures defining CONFIG_ARCH_WANTS_NO_INSTR, Tasks
> > > RCU should instead check the ct_kernel_enter_state(RCU_DYNTICKS_IDX)
> > > state, right?  And on those architectures, I believe that
> > > rcu_dynticks_task_enter() and rcu_dynticks_task_exit() can just be no-ops.
> > > Or am I missing something here?
> > 
> > I think rcu_dynticks_task_enter() and rcu_dynticks_task_exit() are
> > still needed anyway because the target task can migrate. So unless the rq is locked,
> > it's hard to match a stable task_cpu() with the corresponding RCU_DYNTICKS_IDX.
> 
> Can it really migrate while in entry/exit or deep idle code?  Or am I
> missing a trick here?

No but it can migrate before or after EQS. So we need to handle situations like:

 == CPU 0 ==                      == CPU 1 ==
                                  // TASK A is on rq
                                  set_task_cpu(TASK A, 0)
// TASK B runs
ct_user_enter()
ct_user_exit()

//TASK A runs


It could be something like the following:


int rcu_tasks_nohz_full_holdout(struct task_struct *t)
{
	int cpu;
	int snap;

	cpu = task_cpu(t);

	/* Don't miss EQS exit if the task migrated out and in */
	smp_rmb()

	snap = ct_dynticks_cpu(cpu);
	if (snap & RCU_DYNTICKS_IDX)
		return true;

	/* Check if it's the actual task running */
	smp_rmb()

	if (rcu_dereference_raw(cpu_curr(cpu)) != t)
		return true;

	/* Make sure the task hasn't migrated in after the above EQS */
	smp_rmb()

	
	return ct_dynticks_cpu(cpu) != snap;
}

But there is still a risk that ct_dynticks wraps before the last test. So
we would need task_call_func() if task_cpu() is in nohz_full mode.

> 
> > > > 2) Looking further into CONFIG_TASKS_TRACE_RCU_READ_MB=y, it seems to
> > > >    allow for uses of rcu_read_[un]lock_trace() while RCU is not watching
> > > >    (EQS). Is it really a good idea to support that? Are we aware of any
> > > >    such potential usecase?
> > > 
> > > I hope that in the longer term, there will be no reason to support this.
> > > Right now, architectures not defining CONFIG_ARCH_WANTS_NO_INSTR must
> > > support this because tracers really can attach probes where RCU is
> > > not watching.
> > > 
> > > And even now, in architectures defining CONFIG_ARCH_WANTS_NO_INSTR, I
> > > am not convinced that the early incoming and late outgoing CPU-hotplug
> > > paths are handled correctly.  RCU is not watching them, but I am not so
> > > sure that they are all marked noinstr as needed.
> > 
> > Ok I see...
> 
> If need be, the outgoing-CPU transition to RCU-not-watching could be
> delayed into arch-specific code.  We already allow this for the incoming
> transition.

That's a lot of scary architectures code to handle :-)
And how do we determine which place is finally safe to stop watching?

Thanks.

> 
> 							Thanx, Paul