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

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Date:   Wed, 8 Aug 2018 19:30:13 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Josh Triplett <josh@...htriplett.org>
Cc:     Michal Hocko <mhocko@...nel.org>, akpm@...ux-foundation.org,
        gregkh@...uxfoundation.org, rafael@...nel.org,
        viro@...iv.linux.org.uk, darrick.wong@...cle.com,
        paulmck@...ux.vnet.ibm.com, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        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,
        penguin-kernel@...ove.SAKURA.ne.jp, aryabinin@...tuozzo.com,
        willy@...radead.org, ying.huang@...el.com, shakeelb@...gle.com,
        jbacik@...com, mingo@...nel.org, mhiramat@...nel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH RFC 01/10] rcu: Make CONFIG_SRCU unconditionally enabled

On 08.08.2018 19:23, Kirill Tkhai wrote:
> On 08.08.2018 19:13, Josh Triplett wrote:
>> On Wed, Aug 08, 2018 at 01:17:44PM +0300, Kirill Tkhai wrote:
>>> 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
>>
>> 41k is a *substantial* size increase. However, can you compare
>> tinyconfig with and without this patch? That may have a smaller change.
> 
> $ size image.srcu.disabled
>    text	   data	    bss	    dec	    hex	filename
> 1105900	 195456	  63232	1364588	 14d26c	image.srcu.disabled
> 
> $ size image.srcu.enabled
>    text	   data	    bss	    dec	    hex	filename
> 1106960	 195528	  63232	1365720	 14d6d8	image.srcu.enabled
> 
> 1365720-1364588 = 1132 ~ 1Kb
 
1Kb is not huge size. It looks as not a big price for writing generic code
for only case (now some places have CONFIG_SRCU and !CONFIG_SRCU variants,
e.g. drivers/base/core.c). What do you think?