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 F13BFC433E9 for ; Wed, 27 Jan 2021 08:17:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B3D4D20758 for ; Wed, 27 Jan 2021 08:17:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234762AbhA0IRI (ORCPT ); Wed, 27 Jan 2021 03:17:08 -0500 Received: from mx2.suse.de ([195.135.220.15]:38672 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232896AbhA0IPE (ORCPT ); Wed, 27 Jan 2021 03:15:04 -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=1611735249; 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=E5AhEGs1fZHdUiGxkSymBk7PBZi4G6XOuFjXSG9Eq4o=; b=mej6njj16e/xkk21Dx/xEB1pgXTMf6sb6IneUutMWu6NULauH+X2idainwluKw5kdb5uVo cnfJSayyz1mlydC/QVOOa742m4v2mXUvJNigoop4fq7aX9g80y5KQUb5lST8ZCZZleG7Fe QwZ1ieA6PznUBPI95WrQItCnV8n757A= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 53A5FB73C; Wed, 27 Jan 2021 08:14:09 +0000 (UTC) Date: Wed, 27 Jan 2021 09:14:08 +0100 From: Michal Hocko To: Minchan Kim Cc: Andrew Morton , linux-mm , LKML , hyesoo.yu@samsung.com, david@redhat.com, surenb@google.com, pullip.cho@samsung.com, joaodias@google.com, hridya@google.com, john.stultz@linaro.org, sumit.semwal@linaro.org, linux-media@vger.kernel.org, devicetree@vger.kernel.org, hch@infradead.org, robh+dt@kernel.org, linaro-mm-sig@lists.linaro.org Subject: Re: [PATCH v4 2/4] mm: failfast mode with __GFP_NORETRY in alloc_contig_range Message-ID: <20210127081408.GA827@dhcp22.suse.cz> References: <20210121175502.274391-1-minchan@kernel.org> <20210121175502.274391-3-minchan@kernel.org> <20210125131200.GG827@dhcp22.suse.cz> <20210126074449.GA827@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: devicetree@vger.kernel.org On Tue 26-01-21 11:10:18, Minchan Kim wrote: > On Tue, Jan 26, 2021 at 08:44:49AM +0100, Michal Hocko wrote: > > On Mon 25-01-21 11:33:36, Minchan Kim wrote: > > > On Mon, Jan 25, 2021 at 02:12:00PM +0100, Michal Hocko wrote: > > > > On Thu 21-01-21 09:55:00, Minchan Kim wrote: > > > > > Contiguous memory allocation can be stalled due to waiting > > > > > on page writeback and/or page lock which causes unpredictable > > > > > delay. It's a unavoidable cost for the requestor to get *big* > > > > > contiguous memory but it's expensive for *small* contiguous > > > > > memory(e.g., order-4) because caller could retry the request > > > > > in different range where would have easy migratable pages > > > > > without stalling. > > > > > > > > > > This patch introduce __GFP_NORETRY as compaction gfp_mask in > > > > > alloc_contig_range so it will fail fast without blocking > > > > > when it encounters pages needed waiting. > > > > > > > > I am not against controling how hard this allocator tries with gfp mask > > > > but this changelog is rather void on any data and any user. > > > > > > > > It is also rather dubious to have retries when then caller says to not > > > > retry. > > > > > > Since max_tries is 1 with ++tries, it shouldn't retry. > > > > OK, I have missed that. This is a tricky code. ASYNC mode should be > > completely orthogonal to the retries count. Those are different things. > > Page allocator does an explicit bail out based on __GFP_NORETRY. You > > should be doing the same. > > A concern with __GFP_NOWAIT is regardless of flags passed to cma_alloc, > internal implementation of alloc_contig_range inside will use blockable > operation. See __alloc_contig_migrate_range. Yes it is now. But nothing should prevent from making it non blockable. > If we go with __GFP_NOWAIT, we should propagate the gfp_mask inside of > __alloc_contig_migrate_range to make cma_alloc consistent with alloc_pages. Absolutely. You should be doing that anyway. As I've said above you shouldn't rely on side effects like ASYNC mode. > (IIUC, that's what you want - make gfp_mask consistent between cma_alloc > and alloc_pages) but I am worry about the direction will make complicate > situation since cma invovles migration context as well as target page > allocation context. Sometime, the single gfp flag could be trouble > to express both contexts all at once. I am not sure I see your concern. > > > > Also why didn't you consider GFP_NOWAIT semantic for non blocking mode? > > > > > > GFP_NOWAIT seems to be low(specific) flags rather than the one I want to > > > express. Even though I said only page writeback/lock in the description, > > > the goal is to avoid costly operations we might find later so such > > > "failfast", I thought GFP_NORETRY would be good fit. > > > > I suspect you are too focused on implementation details here. Think > > about the indended semantic. Callers of this functionality will not > > think about those (I hope because if they rely on these details then the > > whole thing will become unmaintainable because any change would require > > an audit of all existing users). All you should be caring about is to > > control how expensive the call can be. GFP_NOWAIT is not really low > > level from that POV. It gives you a very lightweight non-sleeping > > attempt to allocate. GFP_NORETRY will give you potentially sleeping but > > an opportunistic-easy-to-fail attempt. And so on. See how that is > > absolutely free of any page writeback or any specific locking. > > With above reason I mentioned, I wanted to express __GFP_NORETRY as > "opportunistic-easy-to-fail attempt" to support cma_alloc as "failfast" > for migration context. Yes that is fine. And please note that I do not push for NOWAIT semantic. If there is no user for that now then fine. -- Michal Hocko SUSE Labs