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

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Message-ID: <20170411173900.00f4b6c6@gandalf.local.home>
Date:   Tue, 11 Apr 2017 17:39:00 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: There is a Tasks RCU stall warning

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.

-- Steve