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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 22A13CDB465 for ; Mon, 16 Oct 2023 10:18:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233015AbjJPKSZ (ORCPT ); Mon, 16 Oct 2023 06:18:25 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60796 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233099AbjJPKSG (ORCPT ); Mon, 16 Oct 2023 06:18:06 -0400 Received: from out-202.mta1.migadu.com (out-202.mta1.migadu.com [95.215.58.202]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E19F9189 for ; Mon, 16 Oct 2023 03:17:34 -0700 (PDT) Message-ID: <121772bf-4c1d-3d23-f266-60ce2e879193@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1697451452; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=IPtMNZC738Ao/n0cWIsZR42e+XNPTKi4tThatun01tI=; b=J4J6IRfAUnzCMQ6IGo3qSSlLyzujeuqcMI/me+pXlw8MI9NJsdz7Xh4dcLf1SOyQVJbv6/ JCnT6o6UwTdpfzYxDKUWZNmkeDibqcWGxrhCsuz9/UnMjdaYbZnxy8WL0mhfE9XWFqOni/ KYKmD15AhrLuIBfAiSOjycN5CZbi93A= Date: Mon, 16 Oct 2023 18:17:23 +0800 MIME-Version: 1.0 Subject: Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when MEMINIT_EARLY Content-Language: en-US To: David Hildenbrand , Mike Rapoport Cc: akpm@linux-foundation.org, mike.kravetz@oracle.com, muchun.song@linux.dev, willy@infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20230928083302.386202-1-yajun.deng@linux.dev> <20230928083302.386202-3-yajun.deng@linux.dev> <20230929083018.GU3303@kernel.org> <2f8c4741-5c7f-272d-9cef-9fda9fbc7ca6@linux.dev> <5382bf2d-5aa0-1498-8169-3248be4b5af3@linux.dev> <38cd0cb9-efe9-b98a-2768-ccb48da8b812@linux.dev> <20231013084827.GT3303@kernel.org> <1c91dd62-886d-bb05-8aee-22191a8a2d8e@linux.dev> <20231016063357.GU3303@kernel.org> <0d890048-be58-5050-02fa-21768059aa0d@linux.dev> <23302f67-eb69-265a-ab2d-9c55715e2843@redhat.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yajun Deng In-Reply-To: <23302f67-eb69-265a-ab2d-9c55715e2843@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2023/10/16 16:36, David Hildenbrand wrote: > On 16.10.23 10:32, Yajun Deng wrote: >> >> On 2023/10/16 16:16, David Hildenbrand wrote: >>> On 16.10.23 10:10, Yajun Deng wrote: >>>> >>>> On 2023/10/16 14:33, Mike Rapoport wrote: >>>>> On Fri, Oct 13, 2023 at 05:29:19PM +0800, Yajun Deng wrote: >>>>>> On 2023/10/13 16:48, Mike Rapoport wrote: >>>>>>> On Thu, Oct 12, 2023 at 05:53:22PM +0800, Yajun Deng wrote: >>>>>>>> On 2023/10/12 17:23, David Hildenbrand wrote: >>>>>>>>> On 10.10.23 04:31, Yajun Deng wrote: >>>>>>>>>> On 2023/10/8 16:57, Yajun Deng wrote: >>>>>>>>>>>> That looks wrong. if the page count would by pure luck be 0 >>>>>>>>>>>> already for hotplugged memory, you wouldn't clear the reserved >>>>>>>>>>>> flag. >>>>>>>>>>>> >>>>>>>>>>>> These changes make me a bit nervous. >>>>>>>>>>> Is 'if (page_count(page) || PageReserved(page))' be safer? Or >>>>>>>>>>> do I >>>>>>>>>>> need to do something else? >>>>>>>>>>> >>>>>>>>>> How about the following if statement? But it needs to add more >>>>>>>>>> patch >>>>>>>>>> like v1 ([PATCH 2/4] mm: Introduce MEMINIT_LATE context). >>>>>>>>>> >>>>>>>>>> It'll be safer, but more complex. Please comment... >>>>>>>>>> >>>>>>>>>>         if (context != MEMINIT_EARLY || (page_count(page) || >>>>>>>>>> PageReserved(page)) { >>>>>>>>>> >>>>>>>>> Ideally we could make initialization only depend on the context, >>>>>>>>> and not >>>>>>>>> check for count or the reserved flag. >>>>>>>>> >>>>>>>> This link is v1, >>>>>>>> https://lore.kernel.org/all/20230922070923.355656-1-yajun.deng@linux.dev/ >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> If we could make initialization only depend on the context, I'll >>>>>>>> modify it >>>>>>>> based on v1. >>>>>>> Although ~20% improvement looks impressive, this is only >>>>>>> optimization of a >>>>>>> fraction of the boot time, and realistically, how much 56 msec >>>>>>> saves from >>>>>>> the total boot time when you boot a machine with 190G of RAM? >>>>>> There are a lot of factors that can affect the total boot time. 56 >>>>>> msec >>>>>> saves may be insignificant. >>>>>> >>>>>> But if we look at the boot log, we'll see there's a significant >>>>>> time jump. >>>>>> >>>>>> before: >>>>>> >>>>>> [    0.250334] ACPI: PM-Timer IO Port: 0x508 >>>>>> [    0.618994] Memory: 173413056K/199884452K available (18440K >>>>>> kernel code, >>>>>> >>>>>> after: >>>>>> >>>>>> [    0.260229] software IO TLB: area num 32. >>>>>> [    0.563497] Memory: 173413056K/199884452K available (18440K >>>>>> kernel code, >>>>>> Memory: >>>>>> Memory initialization is time consuming in the boot log. >>>>> You just confirmed that 56 msec is insignificant and then you send >>>>> again >>>>> the improvement of ~60 msec in memory initialization. >>>>> >>>>> What does this improvement gain in percentage of total boot time? >>>> >>>> >>>> before: >>>> >>>> [   10.692708] Run /init as init process >>>> >>>> >>>> after: >>>> >>>> [   10.666290] Run /init as init process >>>> >>>> >>>> About 0.25%. The total boot time is variable, depending on how many >>>> drivers need to be initialized. >>>> >>>> >>>>>>> I still think the improvement does not justify the churn, added >>>>>>> complexity >>>>>>> and special casing of different code paths of initialization of >>>>>>> struct pages. >>>>>> >>>>>> Because there is a loop, if the order is MAX_ORDER, the loop will >>>>>> run 1024 >>>>>> times. The following 'if' would be safer: >>>>>> >>>>>> 'if (context != MEMINIT_EARLY || (page_count(page) || >> >>>>>> PageReserved(page)) >>>>>> {' >>>>> No, it will not. >>>>> >>>>> As the matter of fact any condition here won't be 'safer' because it >>>>> makes >>>>> the code more complex and less maintainable. >>>>> Any future change in __free_pages_core() or one of it's callers will >>>>> have >>>>> to reason what will happen with that condition after the change. >>>> >>>> >>>> To avoid introducing MEMINIT_LATE context and make code simpler. This >>>> might be a better option. >>>> >>>> if (page_count(page) || PageReserved(page)) >>> >>> I'll have to side with Mike here; this change might not be worth it. >>> >> >> Okay, I got it. Thanks! > > IMHO instead of adding more checks to that code we should try to unify > that handling such that we can just remove it. As expressed, at least > from the memory hotplug perspective there are still reasons why we > need that; I can provide some guidance on how to eventually achieve > that, but it might end up in a bit of work ... Yes, we can't remove it right now. If we want to do that, we have to clean up rely on page count and PageReserved first. > > Anyhow, thanks for bringing up that topic; it reminded me that I still > have pending cleanups to not rely on PageReserved on the memory > hotplug path. >