From: Michal Hocko <mhocko@kernel.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
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`
Date: Wed, 30 Nov 2016 18:34:34 +0100 [thread overview]
Message-ID: <20161130173433.GL18432@dhcp22.suse.cz> (raw)
In-Reply-To: <20161130172355.GA3924@linux.vnet.ibm.com>
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
WARNING: multiple messages have this Message-ID (diff)
From: Michal Hocko <mhocko@kernel.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
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`
Date: Wed, 30 Nov 2016 18:34:34 +0100 [thread overview]
Message-ID: <20161130173433.GL18432@dhcp22.suse.cz> (raw)
In-Reply-To: <20161130172355.GA3924@linux.vnet.ibm.com>
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
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next prev parent reply other threads:[~2016-11-30 17:34 UTC|newest]
Thread overview: 94+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <24c226a5-1a4a-173e-8b4e-5107a2baac04@molgen.mpg.de>
2016-11-08 12:22 ` INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and `mem_cgroup_shrink_node` Paul Menzel
2016-11-08 17:03 ` Paul E. McKenney
2016-11-08 17:38 ` Paul Menzel
2016-11-08 18:39 ` Paul E. McKenney
2016-11-16 17:01 ` Paul Menzel
2016-11-16 17:30 ` Paul E. McKenney
2016-11-21 13:41 ` Michal Hocko
2016-11-21 14:01 ` Paul E. McKenney
2016-11-21 14:18 ` Michal Hocko
2016-11-21 14:29 ` Paul E. McKenney
2016-11-21 15:35 ` Donald Buczek
2016-11-24 10:15 ` Michal Hocko
2016-11-24 18:50 ` Donald Buczek
2016-11-27 9:37 ` Paul Menzel
2016-11-27 5:32 ` Christopher S. Aker
2016-11-27 9:19 ` Donald Buczek
2016-11-28 11:04 ` Michal Hocko
2016-11-28 12:26 ` Paul Menzel
2016-11-28 12:26 ` Paul Menzel
2016-11-30 10:28 ` Donald Buczek
2016-11-30 10:28 ` Donald Buczek
2016-11-30 11:09 ` Michal Hocko
2016-11-30 11:09 ` Michal Hocko
2016-11-30 11:43 ` Donald Buczek
2016-11-30 11:43 ` Donald Buczek
2016-12-02 9:14 ` Donald Buczek
2016-12-02 9:14 ` Donald Buczek
2016-12-06 8:32 ` Donald Buczek
2016-12-06 8:32 ` Donald Buczek
2016-11-30 11:53 ` Paul E. McKenney
2016-11-30 11:53 ` Paul E. McKenney
2016-11-30 11:54 ` Paul E. McKenney
2016-11-30 11:54 ` Paul E. McKenney
2016-11-30 12:31 ` Paul Menzel
2016-11-30 12:31 ` Paul Menzel
2016-11-30 14:31 ` Paul E. McKenney
2016-11-30 14:31 ` Paul E. McKenney
2016-11-30 13:19 ` Michal Hocko
2016-11-30 13:19 ` Michal Hocko
2016-11-30 14:29 ` Paul E. McKenney
2016-11-30 14:29 ` Paul E. McKenney
2016-11-30 16:38 ` Peter Zijlstra
2016-11-30 16:38 ` Peter Zijlstra
2016-11-30 17:02 ` Paul E. McKenney
2016-11-30 17:02 ` Paul E. McKenney
2016-11-30 17:05 ` Michal Hocko
2016-11-30 17:05 ` Michal Hocko
2016-11-30 17:23 ` Paul E. McKenney
2016-11-30 17:23 ` Paul E. McKenney
2016-11-30 17:34 ` Michal Hocko [this message]
2016-11-30 17:34 ` Michal Hocko
2016-11-30 17:50 ` Peter Zijlstra
2016-11-30 17:50 ` Peter Zijlstra
2016-11-30 19:40 ` Paul E. McKenney
2016-11-30 19:40 ` Paul E. McKenney
2016-12-01 5:30 ` Peter Zijlstra
2016-12-01 5:30 ` Peter Zijlstra
2016-12-01 12:40 ` Paul E. McKenney
2016-12-01 12:40 ` Paul E. McKenney
2016-12-01 16:36 ` Peter Zijlstra
2016-12-01 16:36 ` Peter Zijlstra
2016-12-01 16:59 ` Paul E. McKenney
2016-12-01 16:59 ` Paul E. McKenney
2016-12-01 18:09 ` Peter Zijlstra
2016-12-01 18:09 ` Peter Zijlstra
2016-12-01 18:42 ` Paul E. McKenney
2016-12-01 18:42 ` Paul E. McKenney
2016-12-01 18:49 ` Peter Zijlstra
2016-12-01 18:49 ` Peter Zijlstra
[not found] <d6981bac-8e97-b482-98c0-40949db03ca3@kernelpanic.ru>
[not found] ` <20161124133019.GE3612@linux.vnet.ibm.com>
[not found] ` <de88a72a-f861-b51f-9fb3-4265378702f1@kernelpanic.ru>
[not found] ` <20161125212000.GI31360@linux.vnet.ibm.com>
[not found] ` <20161128095825.GI14788@dhcp22.suse.cz>
[not found] ` <20161128105425.GY31360@linux.vnet.ibm.com>
[not found] ` <3a4242cb-0198-0a3b-97ae-536fb5ff83ec@kernelpanic.ru>
[not found] ` <20161128143435.GC3924@linux.vnet.ibm.com>
2016-11-28 14:40 ` Boris Zhmurov
2016-11-28 15:05 ` Paul E. McKenney
2016-11-28 19:16 ` Boris Zhmurov
2016-11-29 18:59 ` Paul E. McKenney
2016-11-30 17:41 ` Boris Zhmurov
2016-11-30 17:48 ` Michal Hocko
2016-11-30 18:12 ` Boris Zhmurov
2016-11-30 18:25 ` Michal Hocko
2016-11-30 18:26 ` Boris Zhmurov
2016-12-01 18:10 ` Boris Zhmurov
2016-12-01 19:39 ` Paul E. McKenney
2016-12-01 19:39 ` Paul E. McKenney
2016-12-02 9:37 ` Michal Hocko
2016-12-02 9:37 ` Michal Hocko
2016-12-02 13:52 ` Paul E. McKenney
2016-12-02 13:52 ` Paul E. McKenney
2016-12-02 16:39 ` Boris Zhmurov
2016-12-02 16:39 ` Boris Zhmurov
2016-12-02 16:44 ` Paul E. McKenney
2016-12-02 16:44 ` Paul E. McKenney
2016-12-02 17:02 ` Michal Hocko
2016-12-02 17:02 ` Michal Hocko
2016-12-02 17:15 ` Paul E. McKenney
2016-12-02 17:15 ` Paul E. McKenney
2016-11-30 19:42 ` Paul E. McKenney
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