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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

  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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