linux-kernel - Re: There is a Tasks RCU stall warning

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Message-Id: <20170411214443.GH1600@linux.vnet.ibm.com>
Date:   Tue, 11 Apr 2017 14:44:43 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: There is a Tasks RCU stall warning

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@...dmis.org> wrote:
> 
> > On Tue, 11 Apr 2017 17:31:33 -0400
> > Steven Rostedt <rostedt@...dmis.org> 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