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: Mon, 10 Aug 2020 21:25:25 +0200 [thread overview]
Message-ID: <20200810192525.GG4773@dhcp22.suse.cz> (raw)
In-Reply-To: <20200810160739.GA29884@pc636>
On Mon 10-08-20 18:07:39, Uladzislau Rezki wrote:
> > On Sun 09-08-20 22:43:53, Uladzislau Rezki (Sony) wrote:
> > [...]
> > > Limitations and concerns (Main part)
> > > ====================================
> > > The current memmory-allocation interface presents to following
> > > difficulties that this patch is designed to overcome:
> > >
> > > a) If built with CONFIG_PROVE_RAW_LOCK_NESTING, the lockdep will
> > > complain about violation("BUG: Invalid wait context") of the
> > > nesting rules. It does the raw_spinlock vs. spinlock nesting
> > > checks, i.e. it is not legal to acquire a spinlock_t while
> > > holding a raw_spinlock_t.
> > >
> > > Internally the kfree_rcu() uses raw_spinlock_t(in rcu-dev branch)
> > > whereas the "page allocator" internally deals with spinlock_t to
> > > access to its zones. The code also can be broken from higher level
> > > of view:
> > > <snip>
> > > raw_spin_lock(&some_lock);
> > > kfree_rcu(some_pointer, some_field_offset);
> > > <snip>
> >
> > Is there any fundamental problem to make zone raw_spin_lock?
> >
> Good point. Converting a regular spinlock to the raw_* variant can solve
> an issue and to me it seems partly reasonable. Because there are other
> questions if we do it:
>
> a) what to do with kswapd and "wake-up path" that uses sleepable lock:
> wakeup_kswapd() -> wake_up_interruptible(&pgdat->kswapd_wait).
If there is no RT friendly variant for waking up process from the atomic
context then we might need to special case this for the RT tree.
> b) How RT people reacts on it? I guess they will no be happy.
zone->lock should be held for a very limited amount of time.
> As i described before, calling the __get_free_page(0) with 0 as argument
> will solve the (a). How correctly is it? From my point of view the logic
> that bypass the wakeup path should be explicitly defined.
gfp_mask == 0 is GFP_NOWAIT (aka an atomic allocation request) which
doesn't wake up kswapd. So if the wakeup is a problem then this would be
a way to go.
> Or we can enter the allocator with (__GFP_HIGH|__GFP_ATOMIC) that bypass
> the __GFP_KSWAPD_RECLAIM as well.
This would be an alternative which consumes memory reserves. Is this
really needed for the particular case?
>
> Any thoughts here? Please comment.
>
> Having proposed flag will not heart RT latency and solve all concerns.
>
> > > b) If built with CONFIG_PREEMPT_RT. Please note, in that case spinlock_t
> > > is converted into sleepable variant. Invoking the page allocator from
> > > atomic contexts leads to "BUG: scheduling while atomic".
> >
> > [...]
> >
> > > Proposal
> > > ========
> > > 1) Make GFP_* that ensures that the allocator returns NULL rather
> > > than acquire its own spinlock_t. Having such flag will address a and b
> > > limitations described above. It will also make the kfree_rcu() code
> > > common for RT and regular kernel, more clean, less handling corner
> > > cases and reduce the code size.
> >
> > I do not think this is a good idea. Single purpose gfp flags that tend
> > to heavily depend on the current implementation of the page allocator
> > have turned out to be problematic. Users used to misunderstand their
> > meaning resulting in a lot of abuse which was not trivial to remove.
> > This flag seem to fall into exactly this sort of category. If there is a
> > problem in nesting then that should be addressed rather than a new flag
> > exported IMHO. If that is absolutely not possible for some reason then
> > we can try to figure out what to do but that really need a very strong
> > justification.
> >
> 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.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2020-08-10 19:25 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 [this message]
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
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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