Re: tasks-trace RCU: question about grace period forward progress

[Paul E. McKenney]

On Thu, Feb 25, 2021 at 03:20:34PM -0500, Mathieu Desnoyers wrote:
> ----- On Feb 25, 2021, at 1:33 PM, paulmck paulmck@xxxxxxxxxx wrote:
> [...]
> > commit 581f79546b6be406a9c7280b2d3511b60821efe0
> > Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Date:   Thu Feb 25 10:26:00 2021 -0800
> > 
> >    rcu-tasks: Add block comment laying out RCU Tasks Trace design
> >    
> >    This commit adds a block comment that gives a high-level overview of
> >    how RCU tasks trace grace periods progress.  It also adds a note about
> >    how exiting tasks are handles, plus it gives an overview of the memory
> 
> handles -> handled

Good eyes, fixed!

> >    ordering.
> >    
> >    Reported-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >    Reported-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> >    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > 
> > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> > index 17c8ebe..f818357 100644
> > --- a/kernel/rcu/tasks.h
> > +++ b/kernel/rcu/tasks.h
> > @@ -726,6 +726,42 @@ EXPORT_SYMBOL_GPL(show_rcu_tasks_rude_gp_kthread);
> > // flavors, rcu_preempt and rcu_sched.  The fact that RCU Tasks Trace
> > // readers can operate from idle, offline, and exception entry/exit in no
> > // way allows rcu_preempt and rcu_sched readers to also do so.
> > +//
> > +// The implementation uses rcu_tasks_wait_gp(), which relies on function
> > +// pointers in the rcu_tasks structure.  The rcu_spawn_tasks_trace_kthread()
> > +// function sets these function pointers up so that rcu_tasks_wait_gp()
> > +// invokes these functions in this order:
> > +//
> > +// rcu_tasks_trace_pregp_step():
> > +//	Initialize the count of readers and block CPU-hotplug operations.
> > +// rcu_tasks_trace_pertask(), invoked on every non-idle task:
> > +//	Initialize per-task state and attempt to identify an immediate
> > +//	quiescent state for that task, or, failing that, attempt to set
> > +//	that task's .need_qs flag so that that task's next outermost
> > +//	rcu_read_unlock_trace() will report the quiescent state (in which
> > +//	case the count of readers is incremented).  If both attempts fail,
> > +//	the task is added to a "holdout" list.
> > +// rcu_tasks_trace_postscan():
> > +//	Initialize state and attempt to identify an immediate quiescent
> > +//	state as above (but only for idle tasks), unblock CPU-hotplug
> > +//	operations, and wait for an RCU grace period to avoid races with
> > +//	tasks that are in the process of exiting.
> > +// check_all_holdout_tasks_trace(), repeatedly until holdout list is empty:
> > +//	Scans the holdout list, attempting to identify a quiescent state
> > +//	for each task on the list.  If there is a quiescent state, the
> > +//	corresponding task is removed from the holdout list.
> > +// rcu_tasks_trace_postgp():
> > +//	Wait for the count of readers do drop to zero, reporting any stalls.
> > +//	Also execute full memory barriers to maintain ordering with code
> > +//	executing after the grace period.
> > +//
> > +// The exit_tasks_rcu_finish_trace() synchronizes with exiting tasks.
> > +//
> > +// Pre-grace-period update-side code is ordered before the grace
> > +// period via the ->cbs_lock and barriers in rcu_tasks_kthread().
> > +// Pre-grace-period read-side code is ordered before the grace period by
> > +// atomic_dec_and_test() of the count of readers (for IPIed readers) and by
> > +// scheduler context-switch ordering (for locked-down non-running readers).
> 
> The rest looks good, thanks!

Thank you for looking it over!

							Thanx, Paul