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

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Date:   Fri, 13 Oct 2023 11:48:27 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Yajun Deng <yajun.deng@...ux.dev>
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 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?

I still think the improvement does not justify the churn, added complexity
and special casing of different code paths of initialization of struct pages.
 
> @Mike,  By the way,  this code will cost more time:
> 
>                 if (context == MEMINIT_HOTPLUG)
>                         flags = INIT_PAGE_COUNT | INIT_PAGE_RESERVED;
>                 __init_single_page(page, pfn, zone, nid, flags);
> 
> 
> [    0.014999] On node 0, zone DMA32: 31679 pages in unavailable ranges
> [    0.311560] ACPI: PM-Timer IO Port: 0x508
> 
> 
> This code will cost less time:
> 
>                 __init_single_page(page, pfn, zone, nid, 0);
>                 if (context == MEMINIT_HOTPLUG) {
>                         init_page_count(page);
>                         __SetPageReserved(page);
> 
> [    0.014299] On node 0, zone DMA32: 31679 pages in unavailable ranges
> [    0.250223] ACPI: PM-Timer IO Port: 0x508
> 

-- 
Sincerely yours,
Mike.