From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f69.google.com (mail-wm0-f69.google.com [74.125.82.69]) by kanga.kvack.org (Postfix) with ESMTP id 8D8176B0008 for ; Wed, 8 Aug 2018 14:02:28 -0400 (EDT) Received: by mail-wm0-f69.google.com with SMTP id q26-v6so116665wmc.0 for ; Wed, 08 Aug 2018 11:02:28 -0700 (PDT) Received: from relay2-d.mail.gandi.net (relay2-d.mail.gandi.net. [217.70.183.194]) by mx.google.com with ESMTPS id g10-v6si3517443wri.283.2018.08.08.11.02.26 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 08 Aug 2018 11:02:26 -0700 (PDT) Date: Wed, 8 Aug 2018 11:01:55 -0700 From: Josh Triplett Subject: Re: [PATCH RFC 01/10] rcu: Make CONFIG_SRCU unconditionally enabled Message-ID: <20180808180152.GA2480@localhost> References: <153365347929.19074.12509495712735843805.stgit@localhost.localdomain> <153365625652.19074.8434946780002619802.stgit@localhost.localdomain> <20180808072040.GC27972@dhcp22.suse.cz> <20180808161330.GA22863@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Kirill Tkhai Cc: Michal Hocko , akpm@linux-foundation.org, gregkh@linuxfoundation.org, rafael@kernel.org, viro@zeniv.linux.org.uk, darrick.wong@oracle.com, paulmck@linux.vnet.ibm.com, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, hughd@google.com, 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, sfr@canb.auug.org.au, vdavydov.dev@gmail.com, chris@chris-wilson.co.uk, penguin-kernel@I-love.SAKURA.ne.jp, aryabinin@virtuozzo.com, willy@infradead.org, ying.huang@intel.com, shakeelb@google.com, jbacik@fb.com, mingo@kernel.org, mhiramat@kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org 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.