From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Wed, 8 Aug 2018 16:09:42 -0700 From: Josh Triplett Subject: Re: [PATCH RFC 01/10] rcu: Make CONFIG_SRCU unconditionally enabled Message-ID: <20180808230941.GA14356@localhost> References: <153365347929.19074.12509495712735843805.stgit@localhost.localdomain> <153365625652.19074.8434946780002619802.stgit@localhost.localdomain> <20180808072040.GC27972@dhcp22.suse.cz> <20180808161330.GA22863@localhost> <20180808180152.GA2480@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Shakeel Butt Cc: Kirill Tkhai , Michal Hocko , Andrew Morton , gregkh@linuxfoundation.org, rafael@kernel.org, Alexander Viro , "Darrick J. Wong" , Paul McKenney , Steven Rostedt , mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, Hugh Dickins , shuah@kernel.org, robh@kernel.org, ulf.hansson@linaro.org, aspriel@gmail.com, vivek.gautam@codeaurora.org, robin.murphy@arm.com, joe@perches.com, heikki.krogerus@linux.intel.com, Stephen Rothwell , Vladimir Davydov , Chris Wilson , Tetsuo Handa , Andrey Ryabinin , Matthew Wilcox , Huang Ying , jbacik@fb.com, Ingo Molnar , mhiramat@kernel.org, LKML , linux-fsdevel , Linux MM List-ID: On Wed, Aug 08, 2018 at 04:02:29PM -0700, Shakeel Butt wrote: > On Wed, Aug 8, 2018 at 11:02 AM Josh Triplett wrote: > > > > 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. > > Hi Josh, the motivation behind enabling SRCU is not to simplify the > code in drivers/base/core.c but rather not to introduce similar ifdefs > in mm/vmscan.c for shrinker traversals. Leaving aside the comment someone made about sticking with rwsem for this, I honestly hope that someday the shrinker is optional too. :)