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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 E188EC64E7C for ; Wed, 2 Dec 2020 18:52:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8ABF221D7F for ; Wed, 2 Dec 2020 18:52:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389553AbgLBSv4 (ORCPT ); Wed, 2 Dec 2020 13:51:56 -0500 Received: from mx2.suse.de ([195.135.220.15]:42978 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726996AbgLBSv4 (ORCPT ); Wed, 2 Dec 2020 13:51:56 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1606935069; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=MrETXDccj16ROSTdsfK+oPYaF1D5OYkdWel8AMipPuI=; b=mldBlNabsf0QaJu/CnF2j9hJEHWdLf0HS7pnsoixlHjRLEzPOm3JkdjDMxhO0yDnmX+Rvx e5H8PPKB48ggBsg81uckMgOgu9MdOZA4L5+ZYkaoLLa2Oz2uQBhq2EPJWMA9f0H8fmlPyB n8Hz/FztB29TDZwkdexmfmG2waIh+NQ= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 2725FACF9; Wed, 2 Dec 2020 18:51:09 +0000 (UTC) Date: Wed, 2 Dec 2020 19:51:07 +0100 From: Michal Hocko To: Minchan Kim Cc: David Hildenbrand , Andrew Morton , LKML , linux-mm , hyesoo.yu@samsung.com, willy@infradead.org, iamjoonsoo.kim@lge.com, vbabka@suse.cz, surenb@google.com, pullip.cho@samsung.com, joaodias@google.com, hridya@google.com, sumit.semwal@linaro.org, john.stultz@linaro.org, Brian.Starkey@arm.com, linux-media@vger.kernel.org, devicetree@vger.kernel.org, robh@kernel.org, christian.koenig@amd.com, linaro-mm-sig@lists.linaro.org Subject: Re: [PATCH v2 2/4] mm: introduce cma_alloc_bulk API Message-ID: <20201202185107.GW17338@dhcp22.suse.cz> References: <20201201175144.3996569-1-minchan@kernel.org> <20201201175144.3996569-3-minchan@kernel.org> <8f006a4a-c21d-9db3-5493-fb1cc651b0cf@redhat.com> <20201202154915.GU17338@dhcp22.suse.cz> <20201202164834.GV17338@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 02-12-20 09:54:29, Minchan Kim wrote: > On Wed, Dec 02, 2020 at 05:48:34PM +0100, Michal Hocko wrote: > > On Wed 02-12-20 08:15:49, Minchan Kim wrote: > > > On Wed, Dec 02, 2020 at 04:49:15PM +0100, Michal Hocko wrote: > > [...] > > > > Well, what I can see is that this new interface is an antipatern to our > > > > allocation routines. We tend to control allocations by gfp mask yet you > > > > are introducing a bool parameter to make something faster... What that > > > > really means is rather arbitrary. Would it make more sense to teach > > > > cma_alloc resp. alloc_contig_range to recognize GFP_NOWAIT, GFP_NORETRY resp. > > > > GFP_RETRY_MAYFAIL instead? > > > > > > If we use cma_alloc, that interface requires "allocate one big memory > > > chunk". IOW, return value is just struct page and expected that the page > > > is a big contiguos memory. That means it couldn't have a hole in the > > > range. > > > However the idea here, what we asked is much smaller chunk rather > > > than a big contiguous memory so we could skip some of pages if they are > > > randomly pinned(long-term/short-term whatever) and search other pages > > > in the CMA area to avoid long stall. Thus, it couldn't work with exising > > > cma_alloc API with simple gfp_mak. > > > > I really do not see that as something really alient to the cma_alloc > > interface. All you should care about, really, is what size of the object > > you want and how hard the system should try. If you have a problem with > > an internal implementation of CMA and how it chooses a range and deal > > with pinned pages then it should be addressed inside the CMA allocator. > > I suspect that you are effectivelly trying to workaround those problems > > by a side implementation with a slightly different API. Or maybe I still > > do not follow the actual problem. > > > > > > I am not deeply familiar with the cma allocator so sorry for a > > > > potentially stupid question. Why does a bulk interface performs better > > > > than repeated calls to cma_alloc? Is this because a failure would help > > > > to move on to the next pfn range while a repeated call would have to > > > > deal with the same range? > > > > > > Yub, true with other overheads(e.g., migration retrial, waiting writeback > > > PCP/LRU draining IPI) > > > > Why cannot this be implemented in the cma_alloc layer? I mean you can > > cache failed cases and optimize the proper pfn range search. > > So do you suggest this? > > enum cma_alloc_mode { > CMA_ALLOC_NORMAL, > CMA_ALLOC_FAIL_FAST, > }; > > struct page *cma_alloc(struct cma *cma, size_t count, unsigned int > align, enum cma_alloc_mode mode); > > >From now on, cma_alloc will keep last failed pfn and then start to > search from the next pfn for both CMA_ALLOC_NORMAL and > CMA_ALLOC_FAIL_FAST if requested size from the cached pfn is okay > within CMA area and then wraparound it couldn't find right pages > from the cached pfn. Othewise, the cached pfn will reset to the zero > so that it starts the search from the 0. I like the idea since it's > general improvement, I think. Yes something like that. There are more options to be clever here - e.g. track ranges etc. but I am not sure this is worth the complexity. > Furthemore, With CMA_ALLOC_FAIL_FAST, it could avoid several overheads > at the cost of sacrificing allocation success ratio like GFP_NORETRY. I am still not sure a specific flag is a good interface. Really can this be gfp_mask instead? > I think that would solve the issue with making the API more flexible. > Before diving into it, I'd like to confirm we are on same page. > Please correct me if I misunderstood. I am not sure you are still thinking about a bulk interface. -- Michal Hocko SUSE Labs