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From: Thomas Gleixner <tglx@linutronix.de>
To: Uladzislau Rezki <urezki@gmail.com>, Michal Hocko <mhocko@suse.com>
Cc: paulmck@kernel.org, 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 15:22:00 +0200	[thread overview]
Message-ID: <874kp6llzb.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20200813095840.GA25268@pc636>

Uladzislau Rezki <urezki@gmail.com> writes:
> On Thu, Aug 13, 2020 at 09:50:27AM +0200, Michal Hocko wrote:
>> On Wed 12-08-20 02:13:25, Thomas Gleixner wrote:
>> [...]
>> > I can understand your rationale and what you are trying to solve. So, if
>> > we can actually have a distinct GFP variant:
>> > 
>> >   GFP_I_ABSOLUTELY_HAVE_TO_DO_THAT_AND_I_KNOW_IT_CAN_FAIL_EARLY
>> 
>> Even if we cannot make the zone->lock raw I would prefer to not
>> introduce a new gfp flag. Well we can do an alias for easier grepping
>> #define GFP_RT_SAFE	0

Just using 0 is sneaky but yes, that's fine :)

Bikeshedding: GFP_RT_NOWAIT or such might be more obvious.

>> that would imply nowait semantic and would exclude waking up kswapd as
>> well. If we can make wake up safe under RT then the alias would reflect
>> that without any code changes.

It basically requires to convert the wait queue to something else. Is
the waitqueue strict single waiter?

>> The second, and the more important part, would be to bail out anytime
>> the page allocator is to take a lock which is not allowed in the current
>> RT context. Something like

>> +	/*
>> +	 * Hard atomic contexts are not supported by the allocator for
>> +	 * anything but pcp requests
>> +	 */
>> +	if (!preemtable())

If you make that preemtible() it might even compile, but that still wont
work because if CONFIG_PREEMPT_COUNT=n then preemptible() is always
false.

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.

>> > The page allocator allocations should also have a limit on the number of
>> > pages and eventually also page order (need to stare at the code or let
>> > Michal educate me that the order does not matter).
>> 
>> In practice anything but order 0 is out of question because we need
>> zone->lock for that currently. Maybe we can introduce pcp lists for
>> higher orders in the future - I have a vague recollection Mel was
>> playing with that some time ago.

Ok.
 
>> > To make it consistent the same GFP_ variant should allow the slab
>> > allocator go to the point where the slab cache is exhausted.
>> > 
>> > Having a distinct and clearly defined GFP_ variant is really key to
>> > chase down offenders and to make reviewers double check upfront why this
>> > is absolutely required.
>> 
>> Having a high level and recognizable gfp mask is OK but I would really
>> like not to introduce a dedicated flag. The page allocator should be
>> able to recognize the context which cannot be handled.

The GFP_xxx == 0 is perfectly fine.

> Sorry for jumping in. We can rely on preemptable() for sure, if CONFIG_PREEMPT_RT
> is enabled, something like below:
>
> if (IS_ENABLED_RT && preemptebale())

Ha, you morphed preemtable() into preemptebale() which will not compile
either :)

Thanks,

        tglx

  parent reply	other threads:[~2020-08-13 13:22 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 [this message]
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
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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