From: "Paul E. McKenney" <email@example.com> To: Michal Hocko <firstname.lastname@example.org> Cc: Thomas Gleixner <email@example.com>, Uladzislau Rezki <firstname.lastname@example.org>, LKML <email@example.com>, RCU <firstname.lastname@example.org>, email@example.com, Andrew Morton <firstname.lastname@example.org>, Vlastimil Babka <email@example.com>, Matthew Wilcox <firstname.lastname@example.org>, "Theodore Y . Ts'o" <email@example.com>, Joel Fernandes <firstname.lastname@example.org>, Sebastian Andrzej Siewior <email@example.com>, Oleksiy Avramchenko <firstname.lastname@example.org>, Peter Zijlstra <email@example.com> Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag Date: Thu, 13 Aug 2020 09:04:42 -0700 [thread overview] Message-ID: <20200813160442.GV4295@paulmck-ThinkPad-P72> (raw) In-Reply-To: <20200813155412.GP9477@dhcp22.suse.cz> On Thu, Aug 13, 2020 at 05:54:12PM +0200, Michal Hocko 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 <firstname.lastname@example.org> 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. 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. Of course, if it is just me being confused, by all means let's give this a shot!!! > [...] > > > > 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. And I am plenty happy if an existing flag or set of flags (up to and including the all-zeroes empty set) can be used in this case. Thanx, Paul
next prev parent reply other threads:[~2020-08-13 16:04 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 [this message] 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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