From: Joel Fernandes <joel@joelfernandes.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Uladzislau Rezki <urezki@gmail.com>,
LKML <linux-kernel@vger.kernel.org>,
linux-mm <linux-mm@kvack.org>,
Andrew Morton <akpm@linux-foundation.org>,
"Theodore Y . Ts'o" <tytso@mit.edu>,
Matthew Wilcox <willy@infradead.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 15:37:33 -0400 [thread overview]
Message-ID: <CAEXW_YQmrfbaDocsc7bLULRR1yUv4=MMMEw3b0s1mXM8sEVVzQ@mail.gmail.com> (raw)
In-Reply-To: <20200504190138.GU2869@paulmck-ThinkPad-P72>
Hi Paul,
On Mon, May 4, 2020 at 3:01 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Mon, May 04, 2020 at 02:08:05PM -0400, Joel Fernandes wrote:
> > 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:
> > [..]
> > > > > > 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?
> >
> > Just to clarify for me, what is the disadvantage of using llist here?
>
> Are there some llist APIs that are not set up for concurrency? I am
> not seeing any.
llist deletion racing with another llist deletion will need locking.
So strictly speaking, some locking is possible with llist usage?
The locklessness as I understand comes when adding and deleting at the
same time. For that no lock is needed. But in the current patch, it
locks anyway to avoid the lost-update of the size of the list.
> The overhead isn't that much of a concern, given that these are not on the
> hotpath, but people reading the code and seeing the cmpxchg operations
> might be forgiven for believing that there is some concurrency involved
> somewhere.
>
> Or am I confused and there are now single-threaded add/delete operations
> for llist?
I do see some examples of llist usage with locking in the kernel code.
One case is: do_init_module() calling llist_add to add to the
init_free_list under module_mutex.
> > Since we don't care about traversing backwards, isn't it better to use llist
> > for this usecase?
> >
> > I think Vlad is using locking as we're also tracking the size of the llist to
> > know when to free pages. This tracking could suffer from the lost-update
> > problem without any locking, 2 lockless llist_add happened simulatenously.
> >
> > Also if list_head is used, it will take more space and still use locking.
>
> Indeed, it would be best to use a non-concurrent singly linked list.
Ok cool :-)
Is it safe to say something like the following is ruled out? ;-) :-D
#define kfree_rcu_list_add llist_add
Thanks,
- Joel
next prev parent reply other threads:[~2020-05-04 19:37 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
2020-05-04 18:08 ` Joel Fernandes
2020-05-04 19:01 ` Paul E. McKenney
2020-05-04 19:37 ` Joel Fernandes [this message]
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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