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

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Message-ID: <20161201163614.GL3092@twins.programming.kicks-ass.net>
Date:   Thu, 1 Dec 2016 17:36:14 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Michal Hocko <mhocko@...nel.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 Thu, Dec 01, 2016 at 04:40:24AM -0800, Paul E. McKenney wrote:
> On Thu, Dec 01, 2016 at 06:30:35AM +0100, Peter Zijlstra wrote:

> > Sure, we all dislike IPIs, but I'm thinking this half-way point is
> > sensible, no point in issuing user visible annoyance if indeed we can
> > prod things back to life, no?
> > 
> > Only if we utterly fail to make it respond should we bug the user with
> > our failure..
> 
> Sold!  ;-)
> 
> I will put together a patch later today.
> 
> My intent is to hold off on the "upgrade cond_resched()" patch, one
> step at a time.  Longer term, I do very much like the idea of having
> cond_resched() do both scheduling and RCU quiescent states, assuming
> that this avoids performance pitfalls.

Well, with the above change cond_resched() is already sufficient, no?

In fact, by doing the IPI thing we get the entire cond_resched*()
family, and we could add the should_resched() guard to
cond_resched_rcu().