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From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Mon, 4 May 2020 14:45:57 +0200
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     "Paul E. McKenney" <paulmck@kernel.org>,
        "Uladzislau Rezki (Sony)" <urezki@gmail.com>,
        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>,
        RCU <rcu@vger.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [PATCH 22/24] rcu/tiny: support reclaim for head-less object
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On Sun, May 03, 2020 at 08:27:00PM -0400, Joel Fernandes wrote:
> On Fri, May 01, 2020 at 04:06:38PM -0700, Paul E. McKenney wrote:
> > On Tue, Apr 28, 2020 at 10:59:01PM +0200, Uladzislau Rezki (Sony) wrote:
> > > Make a kvfree_call_rcu() function to support head-less
> > > freeing. Same as for tree-RCU, for such purpose we store
> > > pointers in array. SLAB and vmalloc ptrs. are mixed and
> > > coexist together.
> > > 
> > > Under high memory pressure it can be that maintaining of
> > > arrays becomes impossible. Objects with an rcu_head are
> > > released via call_rcu(). When it comes to the head-less
> > > variant, the kvfree() call is directly inlined, i.e. we
> > > do the same as for tree-RCU:
> > >     a) wait until a grace period has elapsed;
> > >     b) direct inlining of the kvfree() call.
> > > 
> > > Thus the current context has to follow might_sleep()
> > > annotation. Also please note that for tiny-RCU any
> > > call of synchronize_rcu() is actually a quiescent
> > > state, therefore (a) does nothing.
> > 
> > Please, please, please just do synchronize_rcu() followed by kvfree()
> > for single-argument kfree_rcu() and friends in Tiny RCU.
> > 
> > Way simpler and probably way faster as well.  And given that Tiny RCU
> > runs only on uniprocessor systems, the complexity probably is buying
> > you very little, if anything.
> 
> Agreed.
> 
Cool. Agree also :)

--
Vlad Rezki