From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id F0F63C2BAEE for ; Wed, 11 Mar 2020 17:58:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9130F20739 for ; Wed, 11 Mar 2020 17:58:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730602AbgCKR6G (ORCPT ); Wed, 11 Mar 2020 13:58:06 -0400 Received: from mx2.suse.de ([195.135.220.15]:47484 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730450AbgCKR6G (ORCPT ); Wed, 11 Mar 2020 13:58:06 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 5C5B3AFA1; Wed, 11 Mar 2020 17:58:02 +0000 (UTC) Subject: Re: [PATCH] mm,page_alloc,cma: conditionally prefer cma pageblocks for movable allocations To: Rik van Riel , Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, Roman Gushchin , Qian Cai , Mel Gorman , Anshuman Khandual , Joonsoo Kim References: <20200306150102.3e77354b@imladris.surriel.com> <20200307143849.a2fcb81a9626dad3ee46471f@linux-foundation.org> <2f3e2cde7b94dfdb8e1f0532d1074e07ef675bc4.camel@surriel.com> From: Vlastimil Babka Message-ID: <5ed7f24b-d21b-75a1-ff74-49a9e21a7b39@suse.cz> Date: Wed, 11 Mar 2020 18:58:00 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: <2f3e2cde7b94dfdb8e1f0532d1074e07ef675bc4.camel@surriel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/8/20 2:23 PM, Rik van Riel wrote: > On Sat, 2020-03-07 at 14:38 -0800, Andrew Morton wrote: >> On Fri, 6 Mar 2020 15:01:02 -0500 Rik van Riel >> wrote: > >> > --- a/mm/page_alloc.c >> > +++ b/mm/page_alloc.c >> > @@ -2711,6 +2711,18 @@ __rmqueue(struct zone *zone, unsigned int >> > order, int migratetype, >> > { >> > struct page *page; >> > >> > + /* >> > + * Balance movable allocations between regular and CMA areas by >> > + * allocating from CMA when over half of the zone's free memory >> > + * is in the CMA area. >> > + */ >> > + if (migratetype == MIGRATE_MOVABLE && >> > + zone_page_state(zone, NR_FREE_CMA_PAGES) > >> > + zone_page_state(zone, NR_FREE_PAGES) / 2) { >> > + page = __rmqueue_cma_fallback(zone, order); >> > + if (page) >> > + return page; >> > + } >> > retry: >> > page = __rmqueue_smallest(zone, order, migratetype); >> > if (unlikely(!page)) { >> >> __rmqueue() is a hot path (as much as any per-page operation can be a >> hot path). What is the impact here? > > That is a good question. For MIGRATE_MOVABLE allocations, > most allocations seem to be order 0, which go through the > per cpu pages array, and rmqueue_pcplist, or be order 9. > > For order 9 allocations, other things seem likely to dominate > the allocation anyway, while for order 0 allocations the > pcp list should take away the sting? I agree it should be in the noise. But please do put it behind CONFIG_CMA #ifdef. My x86_64 desktop distro kernel doesn't have CONFIG_CMA. Even if this is effectively no-op with __rmqueue_cma_fallback() returning NULL immediately, I think the compiler cannot eliminate the two zone_page_state()'s which are atomic_long_read(), even if it's just ultimately READ_ONCE() here, that's a volatile cast which means elimination not possible AFAIK? Other architectures might be even more involved. Otherwise I agree this is a reasonable solution until CMA is rewritten. > What I do not know is how much impact this change would > have on other allocations, like order 3 or order 4 network > buffer allocations from irq context... > > Are there cases in particular that we should be testing? >