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From: peterz@infradead.org
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Michal Hocko <mhocko@suse.com>,
	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>
Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag
Date: Fri, 14 Aug 2020 18:19:04 +0200	[thread overview]
Message-ID: <20200814161904.GD2674@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200814141425.GM4295@paulmck-ThinkPad-P72>

On Fri, Aug 14, 2020 at 07:14:25AM -0700, Paul E. McKenney wrote:

> Doing this to kfree_rcu() is the first step.  We will also be doing this
> to call_rcu(), which has some long-standing invocations from various
> raw contexts, including hardirq handler.

Most hardirq handler are not raw on RT due to threaded interrupts.
Lockdep knows about this.

> > >   4) As kfree_rcu() can be used from any context except NMI and RT
> > >      relies on it we ran into a circular dependency problem.
> > 
> > Well, which actual usage sites are under a raw spinlock? Most of the
> > ones I could find are not.
> 
> There are some on their way in, but this same optimization will be applied
> to call_rcu(), and there are no shortage of such call sites in that case.
> And these call sites have been around for a very long time.

Luckily there is lockdep to help you find the ones that need converting
to raw_call_rcu() :-)

> > >   Clearly the simplest solution but not Pauls favourite and
> > >   unfortunately he has a good reason.
> > 
> > Which isn't actually stated anywhere I suppose ?
> 
> Several times, but why not one more?  ;-)
> 
> In CONFIG_PREEMPT_NONE=y kernels, which are heavily used, and for which
> the proposed kfree_rcu() and later call_rcu() optimizations are quite
> important, there is no way to tell at runtime whether or you are in
> atomic raw context.

CONFIG_PREEMPT_NONE has nothing what so ever to do with any of this.

> > > > 2.	Adding a GFP_ flag.
> > > 
> > >   Michal does not like the restriction for !RT kernels and tries to
> > >   avoid the introduction of a new allocation mode.
> > 
> > Like above, I tend to be with Michal on this, just wrap the actual
> > allocation in CONFIG_PREEMPT_RT, the code needs to handle a NULL pointer
> > there anyway.
> 
> That disables the optimization in the CONFIG_PREEMPT_NONE=y case,
> where it really is needed.

No, it disables it for CONFIG_PREEMPT_RT.

> I would be OK with either.  In CONFIG_PREEMPT_NONE=n kernels, the
> kfree_rcu() code could use preemptible() to determine whether it was safe
> to invoke the allocator.  The code in kfree_rcu() might look like this:
> 
> 	mem = NULL;
> 	if (IS_ENABLED(CONFIG_PREEMPT_NONE) || preemptible())
> 		mem = __get_free_page(...);
> 
> Is your point is that the usual mistakes would then be caught by the
> usual testing on CONFIG_PREEMPT_NONE=n kernels?

	mem = NULL;
#if !defined(CONFIG_PREEMPT_RT) && !defined(CONFIG_PROVE_LOCKING)
	mem = __get_free_page(...)
#endif
	if (!mem)

But I _really_ would much rather have raw_kfree_rcu() and raw_call_rcu()
variants for the few places that actually need it.

> > >  These are not seperate of each other as #3 requires #4. The most
> > >  horrible solution IMO from a technical POV as it proliferates
> > >  inconsistency for no good reaosn.
> > > 
> > >  Aside of that it'd be solving a problem which does not exist simply
> > >  because kfree_rcu() does not depend on it and we really don't want to
> > >  set precedence and encourage the (ab)use of this in any way.
> > 
> > My preferred solution is 1, if you want to use an allocator, you simply
> > don't get to play under raw_spinlock_t. And from a quick grep, most
> > kfree_rcu() users are not under raw_spinlock_t context.
> 
> There is at least one on its way in, but more to the point, we will
> need to apply this same optimization to call_rcu(), which is used in

There is no need, call_rcu() works perfectly fine today, thank you.
You might want to, but that's an entirely different thing.

> raw atomic context, including from hardirq handlers.

Threaded IRQs. There really is very little code that is 'raw' on RT.

  parent reply	other threads:[~2020-08-14 16:19 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
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 [this message]
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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