From: Michal Hocko <mhocko@suse.com>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: 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>,
"Paul E . McKenney" <paulmck@kernel.org>,
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: Tue, 11 Aug 2020 12:26:49 +0200 [thread overview]
Message-ID: <20200811102649.GI4793@dhcp22.suse.cz> (raw)
In-Reply-To: <20200811093713.GB2634@pc636>
On Tue 11-08-20 11:37:13, Uladzislau Rezki wrote:
> On Tue, Aug 11, 2020 at 10:19:17AM +0200, Michal Hocko wrote:
> > On Mon 10-08-20 21:25:26, Michal Hocko wrote:
> > > On Mon 10-08-20 18:07:39, Uladzislau Rezki wrote:
> > [...]
> > > > The problem that i see is we can not use the page allocator from atomic
> > > > contexts, what is our case:
> > > >
> > > > <snip>
> > > > local_irq_save(flags) or preempt_disable() or raw_spinlock();
> > > > __get_free_page(GFP_ATOMIC);
> > > > <snip>
> > > >
> > > > So if we can convert the page allocator to raw_* lock it will be appreciated,
> > > > at least from our side, IMHO, not from RT one. But as i stated above we need
> > > > to sort raised questions out if converting is done.
> > > >
> > > > What is your view?
> > >
> > > To me it would make more sense to support atomic allocations also for
> > > the RT tree. Having both GFP_NOWAIT and GFP_ATOMIC which do not really
> > > work for atomic context in RT sounds subtle and wrong.
> >
> > I was thinking about this some more. I still think the above would be a
> > reasonable goal we should try to achieve. If for not other then for
> > future maintainability (especially after the RT patchset is merged).
> > I have tried to search for any known problems/attempts to make
> > zone->lock raw but couldn't find anything. Maybe somebody more involved
> > in RT world have something to say about that.
> >
> I tried yesterday to convert zone->lock. See below files i had to modify:
> <snip>
> modified: include/linux/mmzone.h
> modified: mm/compaction.c
> modified: mm/memory_hotplug.c
> modified: mm/page_alloc.c
> modified: mm/page_isolation.c
> modified: mm/page_reporting.c
> modified: mm/shuffle.c
> modified: mm/vmscan.c
> modified: mm/vmstat.c
> <snip>
>
> There is one more lock, that is zone->lru_lock one. Both zone->lock and this
> one intersect between each other. If the lru_lock can be nested under zone->lock
> it should be converted as well. But i need to analyze it farther. There are
> two wrapper functions which are used as common interface to lock/unlock both
> locks. See compact_lock_irqsave()/compact_unlock_should_abort_lru() in the
> mm/compaction.c.
>
> Any thoughts here?
I am not an expert on compaction. Vlastimil would know better. My
thinking was that zone->lock is a tail lock but compaction/page
isolation might be doing something I am not aware of right now.
> Anyway i tried to convert only zone->lock and use page allocator passing there
> gfp_mask=0 as argument. So it works. CONFIG_PROVE_RAW_LOCK_NESTING does not
> complain about any "bad" lock nesting.
>
> > Anyway, if the zone->lock is not a good fit for raw_spin_lock then the
> > only way I can see forward is to detect real (RT) atomic contexts and
> > bail out early before taking the lock in the allocator for NOWAIT/ATOMIC
> > requests.
> >
> For RT kernel we can detect it for sure. preemtable() works just fine there,
> i.e. we can identify the context we are currently in.
In previous email I didn't mention why I prefer full NOWAIT semantic
over rt specific bailouts. There are users making NOWAIT allocation
attempts as an opportunistic allocation request which is OK to fail
as they have a fallback to go through. This would imply they would
prefer to know this ASAP rather then get blocked and sleep. A lack of
reports for PREEMPT_RT would suggest that nobody has noticed as this
though.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2020-08-11 10:26 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 [this message]
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
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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