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

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Message-ID: <20170412105937.43175471@gandalf.local.home>
Date:   Wed, 12 Apr 2017 10:59:37 -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 Wed, 12 Apr 2017 07:48:00 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> > > > Like this?  (Untested, but builds at least some of the time.)  
> > > 
> > > Not like that...  :-/  Update on its way.  
> > 
> > Perhaps more like this.  Started rcutorture on it, will see how it goes.  

I just love the above discussion with yourself ;-)

> 
> Do you need this patch?  If so, I should do some more work on it to
> eliminate the extra common-case branch on the scheduler fastpath.
> 

Do I still need this patch? Maybe. :-)

I changed my benchmark test to call cond_resched_rcu_qs() instead and
that appears to fix the issue. But I'm not sure if there's any other
kthread out there that just calls cond_resched() or schedule().

Actually, I think it is still a good idea to have it. I believe that it
will still allow synchronize_rcu_tasks() to progress even if there's a
kthread task that is constantly being woken up, and never sleeps when
it calls schedule(), as it may always have the R state.

-- Steve