linux-kernel - Re: [PATCH 09/24] rcu/tree: cache specified number of objects

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Message-ID: <20200504180805.GA172409@google.com>
Date:   Mon, 4 May 2020 14:08:05 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Matthew Wilcox <willy@...radead.org>,
        RCU <rcu@...r.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 09/24] rcu/tree: cache specified number of objects

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?

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.

Thoughts?

thanks,

 - Joel

> > 
> > 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 :) 
> 
> --
> Vlad Rezki