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From: "Paul E. McKenney" <paulmck@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Uladzislau Rezki <urezki@gmail.com>,
	Mel Gorman <mgorman@techsingularity.net>,
	Michal Hocko <mhocko@suse.com>,
	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>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Theodore Y . Ts'o" <tytso@mit.edu>,
	Joel Fernandes <joel@joelfernandes.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>,
	Mel Gorman <mgorman@suse.de>
Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.
Date: Thu, 24 Sep 2020 08:38:34 -0700	[thread overview]
Message-ID: <20200924153834.GW29330@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200924111907.GE2628@hirez.programming.kicks-ass.net>

On Thu, Sep 24, 2020 at 01:19:07PM +0200, Peter Zijlstra wrote:
> On Thu, Sep 24, 2020 at 10:16:14AM +0200, Uladzislau Rezki wrote:
> > The key point is "enough". We need pages to make a) fast progress b) support
> > single argument of kvfree_rcu(one_arg). Not vice versa. That "enough" depends
> > on scheduler latency and vague pre-allocated number of pages, it might
> > be not enough what would require to refill it more and more or we can overshoot
> > that would lead to memory overhead. So we have here timing issues and
> > not accurate model. IMHO.
> 
> I'm firmly opposed to the single argument kvfree_rcu() idea, that's
> requiring memory to free memory.

Not quite.

First, there is a fallback when memory allocation fails.  Second,
in heavy-use situations, there is only one allocation per about
500 kvfree_rcu() calls on 64-bit systems.  Third, there are other
long-standing situations that require allocating memory in order to
free memory.

So I agree that it is a good general rule of thumb to avoid allocating
on free paths, but there are exceptions.  This is one of them.

						Thanx, Paul

  parent reply	other threads:[~2020-09-24 15:38 UTC|newest]

Thread overview: 78+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-18 19:48 [PATCH 0/4] kvfree_rcu() and _LOCK_NESTING/_PREEMPT_RT Uladzislau Rezki (Sony)
2020-09-18 19:48 ` [PATCH 1/4] rcu/tree: Add a work to allocate pages from regular context Uladzislau Rezki (Sony)
2020-09-18 19:48 ` [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func Uladzislau Rezki (Sony)
2020-09-21  7:47   ` Michal Hocko
2020-09-21 15:45     ` Paul E. McKenney
2020-09-21 16:03       ` Michal Hocko
2020-09-21 19:48         ` Uladzislau Rezki
2020-09-22  7:50           ` Michal Hocko
2020-09-22 13:12             ` Uladzislau Rezki
2020-09-22 15:35               ` Michal Hocko
2020-09-23 10:37               ` Mel Gorman
2020-09-23 15:41                 ` Paul E. McKenney
2020-09-23 23:22                   ` Mel Gorman
2020-09-24  8:16                     ` Uladzislau Rezki
2020-09-24 11:16                       ` Peter Zijlstra
2020-09-24 15:16                         ` Uladzislau Rezki
2020-09-24 11:19                       ` Peter Zijlstra
2020-09-24 15:21                         ` Uladzislau Rezki
2020-09-25  8:15                           ` Peter Zijlstra
2020-09-25 10:25                             ` Uladzislau Rezki
2020-09-24 15:38                         ` Paul E. McKenney [this message]
2020-09-25  8:26                           ` Peter Zijlstra
2020-09-26 14:37                             ` Paul E. McKenney
2020-09-25  8:05                       ` Michal Hocko
2020-09-25 15:31                         ` Uladzislau Rezki
2020-09-25 15:47                           ` Michal Hocko
2020-09-29 16:25                             ` Uladzislau Rezki
2020-09-30  9:27                               ` Michal Hocko
2020-09-30 12:35                                 ` Uladzislau Rezki
2020-09-30 12:44                                   ` Michal Hocko
2020-09-30 13:39                                     ` Uladzislau Rezki
2020-09-30 16:46                                       ` Michal Hocko
2020-09-30 20:36                                         ` Uladzislau Rezki
2020-09-30 15:25                             ` Joel Fernandes
2020-09-30 16:48                               ` Michal Hocko
2020-09-30 17:03                                 ` Joel Fernandes
2020-09-30 17:03                                   ` Joel Fernandes
2020-09-30 17:22                                   ` Michal Hocko
2020-09-30 17:48                                     ` Joel Fernandes
2020-09-30 17:48                                       ` Joel Fernandes
2020-09-25 16:17                           ` Mel Gorman
2020-09-25 17:57                             ` Uladzislau Rezki
2020-09-22 15:49             ` Paul E. McKenney
2020-09-22  3:35         ` Paul E. McKenney
2020-09-22  8:03           ` Michal Hocko
2020-09-22 15:46             ` Paul E. McKenney
2020-09-23 11:27               ` Uladzislau Rezki
2020-09-29 10:15   ` Vlastimil Babka
2020-09-29 22:07     ` Uladzislau Rezki
2020-09-30 10:35       ` Michal Hocko
2020-10-01 19:32         ` Uladzislau Rezki
2020-09-30 14:39       ` Vlastimil Babka
2020-09-30 15:37         ` Joel Fernandes
2020-10-01 19:26         ` Uladzislau Rezki
2020-10-02  7:11           ` Michal Hocko
2020-10-02  8:50             ` Mel Gorman
2020-10-02  9:05               ` Michal Hocko
2020-10-05 15:08                 ` Uladzislau Rezki
2020-10-05 15:41                   ` Michal Hocko
2020-10-06 22:25                     ` Uladzislau Rezki
2020-10-07 10:02                       ` Michal Hocko
2020-10-07 11:02                         ` Uladzislau Rezki
2020-10-02  9:07               ` Peter Zijlstra
2020-10-02  9:45                 ` Mel Gorman
2020-10-02  9:58                   ` Peter Zijlstra
2020-10-02 10:19                     ` Mel Gorman
2020-10-02 14:41                       ` Paul E. McKenney
2020-10-06 10:03                         ` Mel Gorman
2020-10-06 15:41                           ` Paul E. McKenney
2020-10-05 13:58             ` Uladzislau Rezki
2020-10-02  8:06           ` Mel Gorman
2020-10-05 14:12             ` Uladzislau Rezki
2020-09-18 19:48 ` [PATCH 3/4] rcu/tree: use " Uladzislau Rezki (Sony)
2020-09-18 19:48 ` [PATCH 4/4] rcu/tree: Use schedule_delayed_work() instead of WQ_HIGHPRI queue Uladzislau Rezki (Sony)
2020-09-20 15:06   ` Paul E. McKenney
2020-09-21 13:27     ` Uladzislau Rezki
2020-09-18 22:15 ` [PATCH 0/4] kvfree_rcu() and _LOCK_NESTING/_PREEMPT_RT Paul E. McKenney
2020-09-30 15:52 ` Joel Fernandes

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