linux-kernel - Re: [PATCH RFC 01/10] rcu: Make CONFIG

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Date:   Wed, 8 Aug 2018 13:17:44 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     akpm@...ux-foundation.org, gregkh@...uxfoundation.org,
        rafael@...nel.org, viro@...iv.linux.org.uk,
        darrick.wong@...cle.com, paulmck@...ux.vnet.ibm.com,
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        hughd@...gle.com, shuah@...nel.org, robh@...nel.org,
        ulf.hansson@...aro.org, aspriel@...il.com,
        vivek.gautam@...eaurora.org, robin.murphy@....com, joe@...ches.com,
        heikki.krogerus@...ux.intel.com, sfr@...b.auug.org.au,
        vdavydov.dev@...il.com, chris@...is-wilson.co.uk,
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        linux-mm@...ck.org
Subject: Re: [PATCH RFC 01/10] rcu: Make CONFIG_SRCU unconditionally enabled

On 08.08.2018 10:20, Michal Hocko wrote:
> On Tue 07-08-18 18:37:36, Kirill Tkhai wrote:
>> This patch kills all CONFIG_SRCU defines and
>> the code under !CONFIG_SRCU.
> 
> The last time somebody tried to do this there was a pushback due to
> kernel tinyfication. So this should really give some numbers about the
> code size increase. Also why can't we make this depend on MMU. Is
> anybody else than the reclaim asking for unconditional SRCU usage?

I don't know one. The size numbers (sparc64) are:

$ size image.srcu.disabled 
   text	   data	    bss	    dec	    hex	filename
5117546	8030506	1968104	15116156	 e6a77c	image.srcu.disabled
$ size image.srcu.enabled
   text	   data	    bss	    dec	    hex	filename
5126175	8064346	1968104	15158625	 e74d61	image.srcu.enabled
The difference is: 15158625-15116156 = 42469 ~41Kb

Please, see the measurement details to my answer to Stephen.

> Btw. I totaly agree with Steven. This is a very poor changelog. It is
> trivial to see what the patch does but it is far from clear why it is
> doing that and why we cannot go other ways.
We possibly can go another way, and there is comment to [2/10] about this.
Percpu rwsem may be used instead, the only thing, it is worse, is it will
make shrink_slab() wait unregistering shrinkers, while srcu-based
implementation does not require this. This may be not a big problem.
But, if SRCU is real problem for embedded people, I really don't want they
hate me in the future because of this, so please CC someone if you know :)

Kirill