Re: There is a Tasks RCU stall warning

From: Paul E. McKenney
Date: Tue Apr 11 2017 - 17:44:57 EST


On Tue, Apr 11, 2017 at 05:39:00PM -0400, Steven Rostedt wrote:
> On Tue, 11 Apr 2017 17:34:47 -0400
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> > On Tue, 11 Apr 2017 17:31:33 -0400
> > Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> >
> > > The thread gets created when I enable the benchmark tracepoint. It just
> > > so happens that my test enables *all* tracepoints, which would of
> > > course include this one as well.
> > >
> > > I'll have to look at this code to see why it is getting missed.
> >
> > Yep, this thread never goes to sleep, but will call cond_resched()
> > periodically. This keeps rcu_tasks() from finishing.
> >
> > Should I add a direct "schedule()" in there instead of a
> > cond_resched(), or do you think rcu_tasks should have cond_resched() be
> > a quiescent state as well?
>
> Actually, I believe this found a bug in my trace_event benchmark
> thread. On a preempt kernel, cond_resched() is a nop and expects to
> only be preempted. Calling schedule() directly should fix everything. I
> shouldn't depend on cond_resched() here.

Works for me!

Hopefully it will also work for your computer. :-)

And whew! Glad to see that the stall warnings worked!

Thanx, Paul