Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when MEMINIT_EARLY

[Yajun Deng]

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@xxxxxxxxx/
> > > >
> > > > 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))