From: "Paul E. McKenney" <paulmck@kernel.org>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
"Theodore Y . Ts'o" <tytso@mit.edu>,
Matthew Wilcox <willy@infradead.org>,
Joel Fernandes <joel@joelfernandes.org>,
RCU <rcu@vger.kernel.org>,
Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [PATCH 09/24] rcu/tree: cache specified number of objects
Date: Mon, 4 May 2020 11:07:37 -0700 [thread overview]
Message-ID: <20200504180737.GT2869@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200504174822.GA20446@pc636>
On Mon, May 04, 2020 at 07:48:22PM +0200, Uladzislau Rezki wrote:
> On Mon, May 04, 2020 at 08:24:37AM -0700, Paul E. McKenney wrote:
> > On Mon, May 04, 2020 at 02:43:23PM +0200, Uladzislau Rezki wrote:
> > > On Fri, May 01, 2020 at 02:27:49PM -0700, Paul E. McKenney wrote:
> > > > On Tue, Apr 28, 2020 at 10:58:48PM +0200, Uladzislau Rezki (Sony) wrote:
> > > > > Cache some extra objects per-CPU. During reclaim process
> > > > > some pages are cached instead of releasing by linking them
> > > > > into the list. Such approach provides O(1) access time to
> > > > > the cache.
> > > > >
> > > > > That reduces number of requests to the page allocator, also
> > > > > that makes it more helpful if a low memory condition occurs.
> > > > >
> > > > > A parameter reflecting the minimum allowed pages to be
> > > > > cached per one CPU is propagated via sysfs, it is read
> > > > > only, the name is "rcu_min_cached_objs".
> > > > >
> > > > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > > > > ---
> > > > > kernel/rcu/tree.c | 64 ++++++++++++++++++++++++++++++++++++++++++++---
> > > > > 1 file changed, 60 insertions(+), 4 deletions(-)
> > > > >
> > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > > index 89e9ca3f4e3e..d8975819b1c9 100644
> > > > > --- a/kernel/rcu/tree.c
> > > > > +++ b/kernel/rcu/tree.c
> > > > > @@ -178,6 +178,14 @@ module_param(gp_init_delay, int, 0444);
> > > > > static int gp_cleanup_delay;
> > > > > module_param(gp_cleanup_delay, int, 0444);
> > > > >
> > > > > +/*
> > > > > + * This rcu parameter is read-only, but can be write also.
> > > >
> > > > You mean that although the parameter is read-only, you see no reason
> > > > why it could not be converted to writeable?
> > > >
> > > I added just a note. If it is writable, then we can change the size of the
> > > per-CPU cache dynamically, i.e. "echo 5 > /sys/.../rcu_min_cached_objs"
> > > would cache 5 pages. But i do not have a strong opinion if it should be
> > > writable.
> > >
> > > > If it was writeable, and a given CPU had the maximum numbr of cached
> > > > objects, the rcu_min_cached_objs value was decreased, but that CPU never
> > > > saw another kfree_rcu(), would the number of cached objects change?
> > > >
> > > No. It works the way: unqueue the page from cache in the kfree_rcu(),
> > > whereas "rcu work" will put it back if number of objects < rcu_min_cached_objs,
> > > if >= will free the page.
> >
> > Just to make sure I understand... If someone writes a smaller number to
> > the sysfs variable, the per-CPU caches will be decreased at that point,
> > immediately during that sysfs write? Or are you saying something else?
> >
> This patch defines it as read-only. It defines the minimum threshold that
> controls number of elements in the per-CPU cache. If we decide to make it
> write also, then we will have full of freedom how to define its behavior,
> i.e. it is not defined because it is read only.
And runtime-read-only sounds like an excellent state for it.
> > > > Presumably the list can also be accessed without holding this lock,
> > > > because otherwise we shouldn't need llist...
> > > >
> > > Hm... We increase the number of elements in cache, therefore it is not
> > > lockless. From the other hand i used llist_head to maintain the cache
> > > because it is single linked list, we do not need "*prev" link. Also
> > > we do not need to init the list.
> > >
> > > But i can change it to list_head. Please let me know if i need :)
> >
> > Hmmm... Maybe it is time for a non-atomic singly linked list? In the RCU
> > callback processing, the operations were open-coded, but they have been
> > pushed into include/linux/rcu_segcblist.h and kernel/rcu/rcu_segcblist.*.
> >
> > Maybe some non-atomic/protected/whatever macros in the llist.h file?
> > Or maybe just open-code the singly linked list? (Probably not the
> > best choice, though.) Add comments stating that the atomic properties
> > of the llist functions aren't neded? Something else?
> >
> In order to keep it simple i can replace llist_head by the list_head?
Fine by me!
> > The comments would be a good start. Just to take pity on people seeing
> > the potential for concurrency and wondering how the concurrent accesses
> > actually happen. ;-)
> >
> Sounds like you are kidding me :)
"Only those who have gone too far can possibly tell you how far you
can go!" ;-)
Thanx, Paul
next prev parent reply other threads:[~2020-05-04 18:07 UTC|newest]
Thread overview: 78+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-28 20:58 [PATCH 00/24] Introduce kvfree_rcu(1 or 2 arguments) Uladzislau Rezki (Sony)
2020-04-28 20:58 ` [PATCH 01/24] rcu/tree: Keep kfree_rcu() awake during lock contention Uladzislau Rezki (Sony)
2020-04-28 20:58 ` [PATCH 02/24] rcu/tree: Skip entry into the page allocator for PREEMPT_RT Uladzislau Rezki (Sony)
2020-04-28 20:58 ` [PATCH 03/24] rcu/tree: Use consistent style for comments Uladzislau Rezki (Sony)
2020-05-01 19:05 ` Paul E. McKenney
2020-05-01 20:52 ` Joe Perches
2020-05-03 23:44 ` Joel Fernandes
2020-05-04 0:23 ` Paul E. McKenney
2020-05-04 0:34 ` Joe Perches
2020-05-04 0:41 ` Joel Fernandes
2020-05-03 23:52 ` Joel Fernandes
2020-05-04 0:26 ` Paul E. McKenney
2020-05-04 0:39 ` Joel Fernandes
2020-04-28 20:58 ` [PATCH 04/24] rcu/tree: Repeat the monitor if any free channel is busy Uladzislau Rezki (Sony)
2020-04-28 20:58 ` [PATCH 05/24] rcu/tree: Simplify debug_objects handling Uladzislau Rezki (Sony)
2020-04-28 20:58 ` [PATCH 06/24] rcu/tree: Simplify KFREE_BULK_MAX_ENTR macro Uladzislau Rezki (Sony)
2020-04-28 20:58 ` [PATCH 07/24] rcu/tree: move locking/unlocking to separate functions Uladzislau Rezki (Sony)
2020-04-28 20:58 ` [PATCH 08/24] rcu/tree: Use static initializer for krc.lock Uladzislau Rezki (Sony)
2020-05-01 21:17 ` Paul E. McKenney
2020-05-04 12:10 ` Uladzislau Rezki
2020-04-28 20:58 ` [PATCH 09/24] rcu/tree: cache specified number of objects Uladzislau Rezki (Sony)
2020-05-01 21:27 ` Paul E. McKenney
2020-05-04 12:43 ` Uladzislau Rezki
2020-05-04 15:24 ` Paul E. McKenney
2020-05-04 17:48 ` Uladzislau Rezki
2020-05-04 18:07 ` Paul E. McKenney [this message]
2020-05-04 18:08 ` Joel Fernandes
2020-05-04 19:01 ` Paul E. McKenney
2020-05-04 19:37 ` Joel Fernandes
2020-05-04 19:51 ` Uladzislau Rezki
2020-05-04 20:15 ` joel
2020-05-04 20:16 ` Paul E. McKenney
2020-05-05 11:03 ` Uladzislau Rezki
2020-04-28 20:58 ` [PATCH 10/24] rcu/tree: add rcutree.rcu_min_cached_objs description Uladzislau Rezki (Sony)
2020-05-01 22:25 ` Paul E. McKenney
2020-05-04 12:44 ` Uladzislau Rezki
2020-04-28 20:58 ` [PATCH 11/24] rcu/tree: Maintain separate array for vmalloc ptrs Uladzislau Rezki (Sony)
2020-05-01 21:37 ` Paul E. McKenney
2020-05-03 23:42 ` Joel Fernandes
2020-05-04 0:20 ` Paul E. McKenney
2020-05-04 0:58 ` Joel Fernandes
2020-05-04 2:20 ` Paul E. McKenney
2020-05-04 14:25 ` Uladzislau Rezki
2020-04-28 20:58 ` [PATCH 12/24] rcu/tiny: support vmalloc in tiny-RCU Uladzislau Rezki (Sony)
2020-04-28 20:58 ` [PATCH 13/24] rcu: Rename rcu_invoke_kfree_callback/rcu_kfree_callback Uladzislau Rezki (Sony)
2020-04-28 20:58 ` [PATCH 14/24] rcu: Rename __is_kfree_rcu_offset() macro Uladzislau Rezki (Sony)
2020-04-28 20:58 ` [PATCH 15/24] rcu: Rename kfree_call_rcu() to the kvfree_call_rcu() Uladzislau Rezki (Sony)
2020-04-28 20:58 ` [PATCH 16/24] mm/list_lru.c: Rename kvfree_rcu() to local variant Uladzislau Rezki (Sony)
2020-04-28 20:58 ` [PATCH 17/24] rcu: Introduce 2 arg kvfree_rcu() interface Uladzislau Rezki (Sony)
2020-04-28 20:58 ` [PATCH 18/24] mm/list_lru.c: Remove kvfree_rcu_local() function Uladzislau Rezki (Sony)
2020-04-28 20:58 ` [PATCH 19/24] rcu/tree: Support reclaim for head-less object Uladzislau Rezki (Sony)
2020-05-01 22:39 ` Paul E. McKenney
2020-05-04 0:12 ` Joel Fernandes
2020-05-04 0:28 ` Paul E. McKenney
2020-05-04 0:32 ` Joel Fernandes
2020-05-04 14:21 ` Uladzislau Rezki
2020-05-04 15:31 ` Paul E. McKenney
2020-05-04 16:56 ` Uladzislau Rezki
2020-05-04 17:08 ` Paul E. McKenney
2020-05-04 12:57 ` Uladzislau Rezki
2020-04-28 20:58 ` [PATCH 20/24] rcu/tree: Make kvfree_rcu() tolerate any alignment Uladzislau Rezki (Sony)
2020-05-01 23:00 ` Paul E. McKenney
2020-05-04 0:24 ` Joel Fernandes
2020-05-04 0:29 ` Paul E. McKenney
2020-05-04 0:31 ` Joel Fernandes
2020-05-04 12:56 ` Uladzislau Rezki
2020-04-28 20:59 ` [PATCH 21/24] rcu/tiny: move kvfree_call_rcu() out of header Uladzislau Rezki (Sony)
2020-05-01 23:03 ` Paul E. McKenney
2020-05-04 12:45 ` Uladzislau Rezki
2020-05-06 18:29 ` Uladzislau Rezki
2020-05-06 18:45 ` Paul E. McKenney
2020-05-07 17:34 ` Uladzislau Rezki
2020-04-28 20:59 ` [PATCH 22/24] rcu/tiny: support reclaim for head-less object Uladzislau Rezki (Sony)
2020-05-01 23:06 ` Paul E. McKenney
2020-05-04 0:27 ` Joel Fernandes
2020-05-04 12:45 ` Uladzislau Rezki
2020-04-28 20:59 ` [PATCH 23/24] rcu: Introduce 1 arg kvfree_rcu() interface Uladzislau Rezki (Sony)
2020-04-28 20:59 ` [PATCH 24/24] lib/test_vmalloc.c: Add test cases for kvfree_rcu() Uladzislau Rezki (Sony)
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