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Date:   Wed, 8 Aug 2018 12:59:40 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Stephen Rothwell <sfr@...b.auug.org.au>
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,
        josh@...htriplett.org, 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, vdavydov.dev@...il.com,
        mhocko@...e.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 04:08, Stephen Rothwell wrote:
> Hi Kirill,
> 
> On Tue, 07 Aug 2018 18:37:36 +0300 Kirill Tkhai <ktkhai@...tuozzo.com> wrote:
>>
>> This patch kills all CONFIG_SRCU defines and
>> the code under !CONFIG_SRCU.
>>
>> Signed-off-by: Kirill Tkhai <ktkhai@...tuozzo.com>
> 
> So what sort of overheads (in terms of code size and performance) are
> we adding by having SRCU enabled where it used not to be?

SRCU is unconditionally enabled for x86, so I had to use another arch (sparc64)
to check the size difference. The config, I used to compile, is attached, SRCU
was enabled via:

diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
index 2d58c26bff9a..6e9116e356d4 100644
--- a/arch/sparc/Kconfig
+++ b/arch/sparc/Kconfig
@@ -15,6 +15,7 @@ config SPARC
 	select ARCH_MIGHT_HAVE_PC_PARPORT if SPARC64 && PCI
 	select ARCH_MIGHT_HAVE_PC_SERIO
 	select OF
+	select SRCU
 	select OF_PROMTREE
 	select HAVE_IDE
 	select HAVE_OPROFILE

$ 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

I have not ideas about performance overhead measurements. If you have ideas,
where they may occur, please say. At the first sight, there should not be
a problem, since SRCU is enabled in x86 by default.

View attachment ".config" of type "text/plain" (65783 bytes)

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