linux-kernel - Re: [PATCH 0/2] rcu: Transform kfree_rcu() into kvfree

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Date:   Wed, 7 Feb 2018 10:10:55 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        mingo@...hat.com, cl@...ux.com, penberg@...nel.org,
        rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, rao.shoaib@...cle.com
Subject: Re: [PATCH 0/2] rcu: Transform kfree_rcu() into kvfree_rcu()

On Wed, Feb 07, 2018 at 05:45:13PM +0100, Jesper Dangaard Brouer wrote:
> On Wed, 7 Feb 2018 08:57:00 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
> > To me kvfree() is a special case and should not be used by RCU as a
> > generic function. That would make RCU and MM much more coupled than
> > necessary.
> 
> For the record, I fully agree with Steve here. 
> 
> And being a performance "fanatic" I don't like to have the extra branch
> (and compares) in the free code path... but it's a MM-decision (and
> sometimes you should not listen to "fanatics" ;-))

While free_rcu() is not withut its performance requirements, I think it's
currently dominated by cache misses and not by branches.  By the time RCU
gets to run callbacks, memory is certainly L1/L2 cache-cold and probably
L3 cache-cold.  Also calling the callback functions is utterly impossible
for the branch predictor.