Message-ID: <8a18b38a-9b49-b583-3a54-54fcd843fb9d@linux.dev>
Date:   Tue, 17 Oct 2023 17:58:24 +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 18:17, Yajun Deng wrote:
>
> 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.


How about making __free_pages_core separate, like:

void __init __free_pages_core_early(struct page *page, unsigned int order)
{
         unsigned int nr_pages = 1 << order;

         atomic_long_add(nr_pages, &page_zone(page)->managed_pages);

         if (page_contains_unaccepted(page, order)) {
                 if (order == MAX_ORDER && __free_unaccepted(page))
                         return;

                 accept_page(page, order);
         }

         /*
          * Bypass PCP and place fresh pages right to the tail, primarily
          * relevant for memory onlining.
          */
         __free_pages_ok(page, order, FPI_TO_TAIL);
}

void __free_pages_core(struct page *page, unsigned int order)
{
         unsigned int nr_pages = 1 << order;
         struct page *p = page;
         unsigned int loop;

         /*
          * When initializing the memmap, __init_single_page() sets the 
refcount
          * of all pages to 1 ("allocated"/"not free"). We have to set the
          * refcount of all involved pages to 0.
          */
         prefetchw(p);
         for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
                 prefetchw(p + 1);
                 __ClearPageReserved(p);
                 set_page_count(p, 0);
         }
         __ClearPageReserved(p);
         set_page_count(p, 0);

         __free_pages_core_early(page, order);
}

We only change the caller we need to __free_pages_core_early, it doesn't 
affect other callers.

>
>>
>> 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.
>>

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