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From: "Paul E. McKenney" <paulmck@kernel.org>
To: Michal Hocko <mhocko@suse.com>
Cc: 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 10:27:54 -0700	[thread overview]
Message-ID: <20200813172754.GA4295@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200813171211.GT9477@dhcp22.suse.cz>

On Thu, Aug 13, 2020 at 07:12:11PM +0200, Michal Hocko wrote:
> On Thu 13-08-20 09:29:04, Paul E. McKenney wrote:
> > On Thu, Aug 13, 2020 at 06:13:57PM +0200, Michal Hocko wrote:
> > > On Thu 13-08-20 09:04:42, Paul E. McKenney wrote:
> > > > On Thu, Aug 13, 2020 at 05:54:12PM +0200, Michal Hocko wrote:
> > > [...]
> > > > > 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.
> > > > 
> > > > Perhaps it was just me getting confused, early hour Pacific Time and
> > > > whatever other excuses might apply.  But I thought that you still had
> > > > an objection to GFP_RT_SAFE based on changes in allocator semantics for
> > > > other users.
> > > 
> > > There is still that problem with lockdep complaining about raw->regular
> > > spinlock on !PREEMPT_RT that would need to get resolved somehow. Thomas
> > > is not really keen on adding some lockdep annotation mechanism and
> > > unfortunatelly I do not have a different idea how to get rid of those.
> > 
> > OK.  So the current situation requires a choice between these these
> > alternatives, each of which has shortcomings that have been mentioned
> > earlier in this thread:
> > 
> > 1.	Prohibit invoking allocators from raw atomic context, such
> > 	as when holding a raw spinlock.
> > 
> > 2.	Adding a GFP_ flag.
> 
> Which would implemente a completely new level atomic allocation for all
> preemption models
> 
> > 
> > 3.	Reusing existing GFP_ flags/values/whatever to communicate
> > 	the raw-context information that was to be communicated by
> > 	the new GFP_ flag.
> 
> this would have to be RT specific to not change the semantic for
> existing users. In other words make NOWAIT semantic working for
> RT atomic contexts.
> 
> > 
> > 4.	Making lockdep forgive acquiring spinlocks while holding
> > 	raw spinlocks, but only in CONFIG_PREEMPT_NONE=y kernels.
> 
> and this would have to go along with 3 to remove false positives on !RT.

OK, let's fill in the issues, then:

1.	Prohibit invoking allocators from raw atomic context, such
	as when holding a raw spinlock.

	o	This would prevent an important cache-locality
		optimization.

2.	Adding a GFP_ flag.

	o	Requires a new level atomic allocation for all preemption
		models, namely, confined to the allocator's lockless
		caches.

3.	Reusing existing GFP_ flags/values/whatever to communicate
	the raw-context information that was to be communicated by
	the new GFP_ flag.

	o	There are existing users of all combinations that might
		be unhappy with a change of semantics.

	o	But Michal is OK with this if usage is restricted to RT.
		Except that this requires #4 below:

4.	Making lockdep forgive acquiring spinlocks while holding
	raw spinlocks, but only in CONFIG_PREEMPT_NONE=y kernels.

	o	This would allow latency degradation and other bad coding
		practices to creep in, per Thomas's recent email.

Again, am I missing anything?

							Thanx, Paul

  reply	other threads:[~2020-08-13 17:27 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 [this message]
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
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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