From: Uladzislau Rezki <urezki@gmail.com>
To: Michal Hocko <mhocko@suse.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Uladzislau Rezki <urezki@gmail.com>,
LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
Vlastimil Babka <vbabka@suse.cz>,
Matthew Wilcox <willy@infradead.org>,
"Theodore Y . Ts'o" <tytso@mit.edu>,
Joel Fernandes <joel@joelfernandes.org>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag
Date: Thu, 13 Aug 2020 18:20:47 +0200 [thread overview]
Message-ID: <20200813162047.GA27774@pc636> (raw)
In-Reply-To: <20200813155412.GP9477@dhcp22.suse.cz>
> On Thu 13-08-20 08:41:59, Paul E. McKenney wrote:
> > On Thu, Aug 13, 2020 at 04:53:35PM +0200, Michal Hocko wrote:
> > > On Thu 13-08-20 16:34:57, Thomas Gleixner wrote:
> > > > Michal Hocko <mhocko@suse.com> writes:
> > > > > On Thu 13-08-20 15:22:00, Thomas Gleixner wrote:
> > > > >> It basically requires to convert the wait queue to something else. Is
> > > > >> the waitqueue strict single waiter?
> > > > >
> > > > > I would have to double check. From what I remember only kswapd should
> > > > > ever sleep on it.
> > > >
> > > > That would make it trivial as we could simply switch it over to rcu_wait.
> > > >
> > > > >> So that should be:
> > > > >>
> > > > >> if (!preemptible() && gfp == GFP_RT_NOWAIT)
> > > > >>
> > > > >> which is limiting the damage to those callers which hand in
> > > > >> GFP_RT_NOWAIT.
> > > > >>
> > > > >> lockdep will yell at invocations with gfp != GFP_RT_NOWAIT when it hits
> > > > >> zone->lock in the wrong context. And we want to know about that so we
> > > > >> can look at the caller and figure out how to solve it.
> > > > >
> > > > > Yes, that would have to somehow need to annotate the zone_lock to be ok
> > > > > in those paths so that lockdep doesn't complain.
> > > >
> > > > That opens the worst of all cans of worms. If we start this here then
> > > > Joe programmer and his dog will use these lockdep annotation to evade
> > > > warnings and when exposed to RT it will fall apart in pieces. Just that
> > > > at that point Joe programmer moved on to something else and the usual
> > > > suspects can mop up the pieces. We've seen that all over the place and
> > > > some people even disable lockdep temporarily because annotations don't
> > > > help.
> > >
> > > Hmm. I am likely missing something really important here. We have two
> > > problems at hand:
> > > 1) RT will become broken as soon as this new RCU functionality which
> > > requires an allocation from inside of raw_spinlock hits the RT tree
> > > 2) lockdep splats which are telling us that early because of the
> > > raw_spinlock-> spin_lock dependency.
> >
> > That is a reasonable high-level summary.
> >
> > > 1) can be handled by handled by the bailing out whenever we have to use
> > > zone->lock inside the buddy allocator - essentially even more strict
> > > NOWAIT semantic than we have for RT tree - proposed (pseudo) patch is
> > > trying to describe that.
> >
> > Unless I am missing something subtle, the problem with this approach
> > is that in production-environment CONFIG_PREEMPT_NONE=y kernels, there
> > is no way at runtime to distinguish between holding a spinlock on the
> > one hand and holding a raw spinlock on the other. Therefore, without
> > some sort of indication from the caller, this approach will not make
> > CONFIG_PREEMPT_NONE=y users happy.
>
> If the whole bailout is guarded by CONFIG_PREEMPT_RT specific atomicity
> check then there is no functional problem - GFP_RT_SAFE would still be
> GFP_NOWAIT so functional wise the allocator will still do the right
> thing.
>
> [...]
>
> > > That would require changing NOWAIT/ATOMIC allocations semantic quite
> > > drastically for !RT kernels as well. I am not sure this is something we
> > > can do. Or maybe I am just missing your point.
> >
> > Exactly, and avoiding changing this semantic for current users is
> > precisely why we are proposing some sort of indication to be passed
> > into the allocation request. In Uladzislau's patch, this was the
> > __GFP_NO_LOCKS flag, but whatever works.
>
> As I've tried to explain already, I would really hope we can do without
> any new gfp flags. We are running out of them and they tend to generate
> a lot of maintenance burden. There is a lot of abuse etc. We should also
> not expose such an implementation detail of the allocator to callers
> because that would make future changes even harder. The alias, on the
> othere hand already builds on top of existing NOWAIT semantic and it
> just helps the allocator to complain about a wrong usage while it
> doesn't expose any internals.
>
I know that Matthew and me raised it. We do can handle it without
introducing any flag. I mean just use 0 as argument to the page_alloc(gfp_flags = 0)
i.e. #define __GFP_NO_LOCKS 0
so it will be handled same way how it is done in the "mm: Add __GFP_NO_LOCKS flag"
I can re-spin the RFC patch and send it out for better understanding.
Does it work for you, Michal? Or it is better just to drop the patch here?
--
Vlad Rezki
next prev parent reply other threads:[~2020-08-13 16:20 UTC|newest]
Thread overview: 111+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-09 20:43 [RFC-PATCH 0/2] __GFP_NO_LOCKS Uladzislau Rezki (Sony)
2020-08-09 20:43 ` [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag Uladzislau Rezki (Sony)
2020-08-10 12:31 ` Michal Hocko
2020-08-10 16:07 ` Uladzislau Rezki
2020-08-10 19:25 ` Michal Hocko
2020-08-11 8:19 ` Michal Hocko
2020-08-11 9:37 ` Uladzislau Rezki
2020-08-11 9:42 ` Uladzislau Rezki
2020-08-11 10:28 ` Michal Hocko
2020-08-11 10:45 ` Uladzislau Rezki
2020-08-11 10:26 ` Michal Hocko
2020-08-11 11:33 ` Uladzislau Rezki
2020-08-11 9:18 ` Uladzislau Rezki
2020-08-11 10:21 ` Michal Hocko
2020-08-11 11:10 ` Uladzislau Rezki
2020-08-11 14:44 ` Thomas Gleixner
2020-08-11 15:22 ` Thomas Gleixner
2020-08-12 11:38 ` Thomas Gleixner
2020-08-12 12:01 ` Uladzislau Rezki
2020-08-13 7:18 ` Michal Hocko
2020-08-11 15:33 ` Paul E. McKenney
2020-08-11 15:43 ` Thomas Gleixner
2020-08-11 15:56 ` Sebastian Andrzej Siewior
2020-08-11 16:02 ` Paul E. McKenney
2020-08-11 16:19 ` Paul E. McKenney
2020-08-11 19:39 ` Thomas Gleixner
2020-08-11 21:09 ` Paul E. McKenney
2020-08-12 0:13 ` Thomas Gleixner
2020-08-12 4:29 ` Paul E. McKenney
2020-08-12 8:32 ` Thomas Gleixner
2020-08-12 13:30 ` Paul E. McKenney
2020-08-13 7:50 ` Michal Hocko
2020-08-13 9:58 ` Uladzislau Rezki
2020-08-13 11:15 ` Michal Hocko
2020-08-13 13:27 ` Thomas Gleixner
2020-08-13 13:45 ` Michal Hocko
2020-08-13 14:32 ` Matthew Wilcox
2020-08-13 16:14 ` Thomas Gleixner
2020-08-13 16:22 ` Matthew Wilcox
2020-08-13 13:22 ` Thomas Gleixner
2020-08-13 13:33 ` Michal Hocko
2020-08-13 14:34 ` Thomas Gleixner
2020-08-13 14:53 ` Michal Hocko
2020-08-13 15:41 ` Paul E. McKenney
2020-08-13 15:54 ` Michal Hocko
2020-08-13 16:04 ` Paul E. McKenney
2020-08-13 16:13 ` Michal Hocko
2020-08-13 16:29 ` Paul E. McKenney
2020-08-13 17:12 ` Michal Hocko
2020-08-13 17:27 ` Paul E. McKenney
2020-08-13 18:31 ` peterz
2020-08-13 19:13 ` Michal Hocko
2020-08-13 16:20 ` Uladzislau Rezki [this message]
2020-08-13 16:36 ` Michal Hocko
2020-08-14 11:54 ` Uladzislau Rezki
2020-08-13 17:09 ` Thomas Gleixner
2020-08-13 17:22 ` Michal Hocko
2020-08-14 7:17 ` Michal Hocko
2020-08-14 12:15 ` Uladzislau Rezki
2020-08-14 12:48 ` Michal Hocko
2020-08-14 13:34 ` Paul E. McKenney
2020-08-14 14:06 ` Michal Hocko
2020-08-14 18:01 ` Paul E. McKenney
2020-08-14 23:14 ` Thomas Gleixner
2020-08-14 23:41 ` Paul E. McKenney
2020-08-15 0:43 ` Thomas Gleixner
2020-08-15 3:01 ` Paul E. McKenney
2020-08-15 8:27 ` Peter Zijlstra
2020-08-15 13:03 ` Paul E. McKenney
2020-08-15 8:42 ` Peter Zijlstra
2020-08-15 14:18 ` Paul E. McKenney
2020-08-15 14:23 ` Paul E. McKenney
2020-08-17 8:47 ` Michal Hocko
2020-08-13 18:26 ` peterz
2020-08-13 18:52 ` Paul E. McKenney
2020-08-13 22:06 ` peterz
2020-08-13 23:23 ` Paul E. McKenney
2020-08-13 23:59 ` Thomas Gleixner
2020-08-14 8:30 ` Peter Zijlstra
2020-08-14 10:23 ` peterz
2020-08-14 15:26 ` Paul E. McKenney
2020-08-14 14:14 ` Paul E. McKenney
2020-08-14 16:11 ` Paul E. McKenney
2020-08-14 17:49 ` Peter Zijlstra
2020-08-14 18:02 ` Paul E. McKenney
2020-08-14 19:33 ` Thomas Gleixner
2020-08-14 20:41 ` Paul E. McKenney
2020-08-14 21:52 ` Peter Zijlstra
2020-08-14 23:27 ` Paul E. McKenney
2020-08-14 23:40 ` Thomas Gleixner
2020-08-16 22:56 ` Uladzislau Rezki
2020-08-17 8:28 ` Michal Hocko
2020-08-17 10:36 ` Uladzislau Rezki
2020-08-17 22:28 ` Paul E. McKenney
2020-08-18 7:43 ` Michal Hocko
2020-08-18 13:53 ` Paul E. McKenney
2020-08-18 14:43 ` Thomas Gleixner
2020-08-18 16:13 ` Paul E. McKenney
2020-08-18 16:55 ` Thomas Gleixner
2020-08-18 17:13 ` Paul E. McKenney
2020-08-18 23:26 ` Thomas Gleixner
2020-08-19 23:07 ` Paul E. McKenney
2020-08-18 15:02 ` Michal Hocko
2020-08-18 15:45 ` Uladzislau Rezki
2020-08-18 16:18 ` Paul E. McKenney
2020-08-14 16:19 ` peterz
2020-08-14 18:15 ` Paul E. McKenney
2020-08-13 13:29 ` Uladzislau Rezki
2020-08-13 13:41 ` Michal Hocko
2020-08-13 14:22 ` Uladzislau Rezki
2020-08-09 20:43 ` [PATCH 2/2] rcu/tree: use " Uladzislau Rezki (Sony)
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