Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756182Ab3CSFEp (ORCPT ); Tue, 19 Mar 2013 01:04:45 -0400 Received: from LGEMRELSE1Q.lge.com ([156.147.1.111]:52965 "EHLO LGEMRELSE1Q.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752725Ab3CSFEo (ORCPT ); Tue, 19 Mar 2013 01:04:44 -0400 X-AuditID: 9c93016f-b7bfaae000003db8-71-5147f1eaaf1e Date: Tue, 19 Mar 2013 14:05:03 +0900 From: Joonsoo Kim To: Nicolas Pitre Cc: Russell King , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH] ARM: mm: disable kmap_high_get() for SMP Message-ID: <20130319050503.GA8858@lge.com> References: <1362372667-953-1-git-send-email-iamjoonsoo.kim@lge.com> <20130307081258.GA14839@lge.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-Brightmail-Tracker: AAAAAA== Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3294 Lines: 85 On Thu, Mar 07, 2013 at 07:35:51PM +0900, JoonSoo Kim wrote: > 2013/3/7 Nicolas Pitre : > > On Thu, 7 Mar 2013, Joonsoo Kim wrote: > > > >> Hello, Nicolas. > >> > >> On Tue, Mar 05, 2013 at 05:36:12PM +0800, Nicolas Pitre wrote: > >> > On Mon, 4 Mar 2013, Joonsoo Kim wrote: > >> > > >> > > With SMP and enabling kmap_high_get(), it makes users of kmap_atomic() > >> > > sequential ordered, because kmap_high_get() use global kmap_lock(). > >> > > It is not welcome situation, so turn off this optimization for SMP. > >> > > >> > I'm not sure I understand the problem. > >> > > >> > The lock taken by kmap_high_get() is released right away before that > >> > function returns and therefore this is not actually serializing > >> > anything. > >> > >> Yes, you understand what I want to say correctly. > >> Sorry for bad explanation. > >> > >> Following is reasons why I send this patch with RFC tag. > >> > >> If we have more cpus, performance degration is possible although > >> it is very short time to holding the lock in kmap_high_get(). > >> > >> And kmap has maximum 512 entries(512 * 4K = 2M) and some mobile devices > >> has 2G memory(highmem 1G>), so probability for finding matched entry > >> is approximately < 1/512. This probability can be more decreasing > >> for device which have more memory. So I think that waste time to find > >> matched entry is more than saved time. > >> > >> Above is my humble opinion, so please let me know what I am missing. > > > > Please look at the kmap_high_get() code again. It performs no > > searching at all. What it does is: > > If page is not highmem, it may be already filtered in kmap_atomic(). > So we only consider highmem page. > > For highmem page, it perform searching. > In kmap_high_get(), page_address() is called. > In page_address(), it hash PA and iterate a list for this hashed value. > > And another advantage of disabling ARCH_NEEDS_KMAP_HIGH_GET is > that kmap(), kunmap() works without irq disabled. > > Thanks. Hello, Nicolas. For just confirm, you don't agree with this, right? Thanks. > > > - lock the kmap array against concurrent changes > > > > - if the given page is not highmem, unlock and return NULL > > > > - otherwise increment that page reference count, unlock, and return the > > mapped address for that page. > > > > There is almost zero cost to this function, independently of the number > > of kmap entries, whereas it does save much bigger costs elsewhere when > > it is successful. > > > > > > Nicolas > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > > the body of a message to majordomo@vger.kernel.org > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > Please read the FAQ at http://www.tux.org/lkml/ > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/