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

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Message-ID: <121772bf-4c1d-3d23-f266-60ce2e879193@linux.dev>
Date:   Mon, 16 Oct 2023 18:17:23 +0800
From:   Yajun Deng <yajun.deng@...ux.dev>
To:     David Hildenbrand <david@...hat.com>,
        Mike Rapoport <rppt@...nel.org>
Cc:     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 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.
>