Date: Wed, 30 Nov 2016 18:05:57 +0100
From: Michal Hocko <mhocko@kernel.org>
To: Peter Zijlstra <peterz@infradead.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: <20161130170557.GK18432@dhcp22.suse.cz>
References: <20161124101525.GB20668@dhcp22.suse.cz>
 <583AA50A.9010608@molgen.mpg.de>
 <20161128110449.GK14788@dhcp22.suse.cz>
 <109d5128-f3a4-4b6e-db17-7a1fcb953500@molgen.mpg.de>
 <29196f89-c35e-f79d-8e4d-2bf73fe930df@molgen.mpg.de>
 <20161130110944.GD18432@dhcp22.suse.cz>
 <20161130115320.GO3924@linux.vnet.ibm.com>
 <20161130131910.GF18432@dhcp22.suse.cz>
 <20161130142955.GS3924@linux.vnet.ibm.com>
 <20161130163820.GQ3092@twins.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20161130163820.GQ3092@twins.programming.kicks-ass.net>
User-Agent: Mutt/1.6.0 (2016-04-01)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1153
Lines: 26

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...

-- 
Michal Hocko
SUSE Labs