linux-kernel - Re: [RFC-PATCH 1/2] mm: Add __GFP_NO

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Date:   Thu, 13 Aug 2020 10:27:54 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Uladzislau Rezki <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Joel Fernandes <joel@...lfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

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