Date: Wed, 30 Nov 2016 18:50:16 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Donald Buczek <buczek@molgen.mpg.de>,
        Paul Menzel <pmenzel@molgen.mpg.de>, dvteam@molgen.mpg.de,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Josh Triplett <josh@joshtriplett.org>
Subject: Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and
 `mem_cgroup_shrink_node`
Message-ID: <20161130175015.GR3092@twins.programming.kicks-ass.net>
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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.

See commit:

  4a81e8328d37 ("rcu: Reduce overhead of cond_resched() checks for RCU")

Someone actually wrote down what the problem was.

> > 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...

Right.. now, this is obviously all PREEMPT=n code, which therefore also
implies this is rcu-sched.

Paul, now doesn't rcu-sched, when the grace-period has been long in
coming, try and force it? And doesn't that forcing include prodding CPUs
with resched_cpu() ?

I'm thinking not, because if it did, that would make cond_resched()
actually schedule, which would then call into rcu_note_context_switch()
which would then make RCU progress, no?