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

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Message-Id: <20170412162754.GJ3956@linux.vnet.ibm.com>
Date:   Wed, 12 Apr 2017 09:27:54 -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 Wed, Apr 12, 2017 at 10:59:37AM -0400, Steven Rostedt wrote:
> 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 ;-)

Talking to oneself used to cause passersby to get really bent out of
shape.  But one big benefit of ubiquitous cellphones is that now
people just assume that you are talking on a cellphone that they
cannot see.  ;-)

> > 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.

OK, will optimize it a bit.  When are you planning to get this in?

							Thanx, Paul