Messages in this thread | | | Date | Mon, 16 Oct 2023 16:32:07 +0800 | Subject | Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when MEMINIT_EARLY | From | Yajun Deng <> |
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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!
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