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Date:   Wed, 8 Aug 2018 11:01:55 -0700
From:   Josh Triplett <josh@joshtriplett.org>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
Cc:     Michal Hocko <mhocko@kernel.org>, akpm@linux-foundation.org,
        gregkh@linuxfoundation.org, rafael@kernel.org,
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Subject: Re: [PATCH RFC 01/10] rcu: Make CONFIG_SRCU unconditionally enabled
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On Wed, Aug 08, 2018 at 07:30:13PM +0300, Kirill Tkhai wrote:
> 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?

That's a little more reasonable than 41k, likely because of
CONFIG_TINY_SRCU. That's still not ideal, though. And as far as I can
tell, the *only* two pieces of core code that use SRCU are
drivers/base/core.c and kernel/notifier.c, and the latter is exclusively
code to use notifiers with SRCU, not notifiers wanting to use SRCU
themselves. So, as far as I can tell, this would really just save a
couple of small #ifdef sections in drivers/base/core.c, and I think
those #ifdef sections could be simplified even further. That doesn't
seem worth it at all.