linux-kernel - Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and `mem_cgroup_shrink

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Date:   Wed, 30 Nov 2016 18:34:34 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Donald Buczek <buczek@...gen.mpg.de>,
        Paul Menzel <pmenzel@...gen.mpg.de>, dvteam@...gen.mpg.de,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>
Subject: Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and
 `mem_cgroup_shrink_node`

On Wed 30-11-16 09:23:55, Paul E. McKenney wrote:
> On Wed, Nov 30, 2016 at 06:05:57PM +0100, Michal Hocko wrote:
> > On Wed 30-11-16 17:38:20, Peter Zijlstra wrote:
> > > On Wed, Nov 30, 2016 at 06:29:55AM -0800, Paul E. McKenney wrote:
> > > > We can, and you are correct that cond_resched() does not unconditionally
> > > > supply RCU quiescent states, and never has.  Last time I tried to add
> > > > cond_resched_rcu_qs() semantics to cond_resched(), I got told "no",
> > > > but perhaps it is time to try again.
> > > 
> > > Well, you got told: "ARRGH my benchmark goes all regress", or something
> > > along those lines. Didn't we recently dig out those commits for some
> > > reason or other?
> > > 
> > > Finding out what benchmark that was and running it against this patch
> > > would make sense.
> > > 
> > > Also, I seem to have missed, why are we going through this again?
> > 
> > Well, the point I've brought that up is because having basically two
> > APIs for cond_resched is more than confusing. Basically all longer in
> > kernel loops do cond_resched() but it seems that this will not help the
> > silence RCU lockup detector in rare cases where nothing really wants to
> > schedule. I am really not sure whether we want to sprinkle
> > cond_resched_rcu_qs at random places just to silence RCU detector...
> 
> Just in case there is any doubt on this point, any patch of mine adding
> cond_resched_rcu_qs() functionality to cond_resched() cannot go upstream
> without Peter's Acked-by.

Yeah, that is clear to me. I just wanted to clarify the "why are we
going through this again" part ;)
 
> Or did you have some other solution in mind?

Not really. The fact that cond_resched() cannot silence RCU stall
detector under some circumstances is sad. I believe we shouldn't
have two different APIs to control scheduling and RCU latencies because
that just asks for whack a mole games and some level of confusion...
-- 
Michal Hocko
SUSE Labs