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

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Message-ID: <20231016063357.GU3303@kernel.org>
Date:   Mon, 16 Oct 2023 09:33:57 +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 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 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?
 
> > 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.

-- 
Sincerely yours,
Mike.