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[2003:cb:c73c:9300:8903:bf2e:db72:6527]) by smtp.gmail.com with ESMTPSA id bq1-20020a5d5a01000000b0031ad5fb5a0fsm904987wrb.58.2023.10.16.01.16.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 16 Oct 2023 01:16:52 -0700 (PDT) Message-ID: Date: Mon, 16 Oct 2023 10:16:52 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.15.1 Subject: Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when MEMINIT_EARLY Content-Language: en-US To: Yajun Deng , 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> From: David Hildenbrand Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. -- Cheers, David / dhildenb