linux-kernel - Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when MEMINIT

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Date:   Mon, 16 Oct 2023 16:10:30 +0800
From:   Yajun Deng <yajun.deng@...ux.dev>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
        mike.kravetz@...cle.com, muchun.song@...ux.dev,
        willy@...radead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when
 MEMINIT_EARLY


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))