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From: Uladzislau Rezki <urezki@gmail.com>
To: Michal Hocko <mhocko@suse.com>
Cc: Uladzislau Rezki <urezki@gmail.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	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: Fri, 14 Aug 2020 13:54:04 +0200	[thread overview]
Message-ID: <20200814115404.GA28229@pc636> (raw)
In-Reply-To: <20200813163617.GS9477@dhcp22.suse.cz>

On Thu, Aug 13, 2020 at 06:36:17PM +0200, Michal Hocko wrote:
> On Thu 13-08-20 18:20:47, Uladzislau Rezki wrote:
> > > 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?
> 
> That would change the semantic for GFP_NOWAIT users who decided to drop
> __GFP_KSWAPD_RECLAIM or even use 0 gfp mask right away, right? The point
>
I see your point. Doing GFP_NOWAIT & ~__GFP_KSWAPD_RECLAIM will do something
different what people expect. Right you are.

>
> I am trying to make is that an alias is good for RT because it doesn't
> have any users (because there is no RT atomic user of the allocator)
> currently.
>
Now I see your view. So we can handle RT case by using "RT && !preemptible()",
based on that we can bail out. GFP_ATOMIC and NOWAIT at least will keep same
semantic.

Second, if the CONFIG_PROVE_RAW_LOCK_NESTING is fixed for PREEMPT_COUNT=n,
then it would work. But i am lost here a bit if it is discussable or not.

Thanks!

--
Vlad Rezki

  reply	other threads:[~2020-08-14 11:54 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
2020-08-13 16:36                                       ` Michal Hocko
2020-08-14 11:54                                         ` Uladzislau Rezki [this message]
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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