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 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.
