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

[Yajun Deng <yajun.deng@linux.dev>]

On 2023/9/29 18:02, Mike Rapoport wrote:
> On Fri, Sep 29, 2023 at 05:50:40PM +0800, Yajun Deng wrote:
>> On 2023/9/29 16:30, Mike Rapoport wrote:
>>> On Thu, Sep 28, 2023 at 04:33:02PM +0800, Yajun Deng wrote:
>>>> memmap_init_range() would init page count of all pages, but the free
>>>> pages count would be reset in __free_pages_core(). There are opposite
>>>> operations. It's unnecessary and time-consuming when it's MEMINIT_EARLY
>>>> context.
>>>>
>>>> Init page count in reserve_bootmem_region when in MEMINIT_EARLY context,
>>>> and check the page count before reset it.
>>>>
>>>> At the same time, the INIT_LIST_HEAD in reserve_bootmem_region isn't
>>>> need, as it already done in __init_single_page.
>>>>
>>>> The following data was tested on an x86 machine with 190GB of RAM.
>>>>
>>>> before:
>>>> free_low_memory_core_early()    341ms
>>>>
>>>> after:
>>>> free_low_memory_core_early()    285ms
>>>>
>>>> Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
>>>> ---
>>>> v4: same with v2.
>>>> v3: same with v2.
>>>> v2: check page count instead of check context before reset it.
>>>> v1: https://lore.kernel.org/all/20230922070923.355656-1-yajun.deng@linux.dev/
>>>> ---
>>>>    mm/mm_init.c    | 18 +++++++++++++-----
>>>>    mm/page_alloc.c | 20 ++++++++++++--------
>>>>    2 files changed, 25 insertions(+), 13 deletions(-)
>>>>
>>>> diff --git a/mm/mm_init.c b/mm/mm_init.c
>>>> index 9716c8a7ade9..3ab8861e1ef3 100644
>>>> --- a/mm/mm_init.c
>>>> +++ b/mm/mm_init.c
>>>> @@ -718,7 +718,7 @@ static void __meminit init_reserved_page(unsigned long pfn, int nid)
>>>>    		if (zone_spans_pfn(zone, pfn))
>>>>    			break;
>>>>    	}
>>>> -	__init_single_page(pfn_to_page(pfn), pfn, zid, nid, INIT_PAGE_COUNT);
>>>> +	__init_single_page(pfn_to_page(pfn), pfn, zid, nid, 0);
>>>>    }
>>>>    #else
>>>>    static inline void pgdat_set_deferred_range(pg_data_t *pgdat) {}
>>>> @@ -756,8 +756,8 @@ void __meminit reserve_bootmem_region(phys_addr_t start,
>>>>    			init_reserved_page(start_pfn, nid);
>>>> -			/* Avoid false-positive PageTail() */
>>>> -			INIT_LIST_HEAD(&page->lru);
>>>> +			/* Init page count for reserved region */
>>> Please add a comment that describes _why_ we initialize the page count here.
>> Okay.
>>>> +			init_page_count(page);
>>>>    			/*
>>>>    			 * no need for atomic set_bit because the struct
>>>> @@ -888,9 +888,17 @@ void __meminit memmap_init_range(unsigned long size, int nid, unsigned long zone
>>>>    		}
>>>>    		page = pfn_to_page(pfn);
>>>> -		__init_single_page(page, pfn, zone, nid, INIT_PAGE_COUNT);
>>>> -		if (context == MEMINIT_HOTPLUG)
>>>> +
>>>> +		/* If the context is MEMINIT_EARLY, we will init page count and
>>>> +		 * mark page reserved in reserve_bootmem_region, the free region
>>>> +		 * wouldn't have page count and we will check the pages count
>>>> +		 * in __free_pages_core.
>>>> +		 */
>>>> +		__init_single_page(page, pfn, zone, nid, 0);
>>>> +		if (context == MEMINIT_HOTPLUG) {
>>>> +			init_page_count(page);
>>>>    			__SetPageReserved(page);
>>> Rather than calling init_page_count() and __SetPageReserved() for
>>> MEMINIT_HOTPLUG you can set flags to INIT_PAGE_COUNT | INIT_PAGE_RESERVED
>>> an call __init_single_page() after the check for MEMINIT_HOTPLUG.
>> No, the following code would cost more time than the current code in
>> memmap_init().
>>
>> if (context == MEMINIT_HOTPLUG)
>>
>> 	__init_single_page(page, pfn, zone, nid, INIT_PAGE_COUNT | INIT_PAGE_RESERVED);
>> else
>> 	
>> 	__init_single_page(page, pfn, zone, nid, 0);
> Sorry if I wasn't clear. What I meant was to have something along these lines:
>
> 	enum page_init_flags flags = 0;
>
> 	if (context == MEMINIT_HOTPLUG)
> 		flags = INIT_PAGE_COUNT | INIT_PAGE_RESERVED;
> 	__init_single_page(page, pfn, zone, nid, flags);
>
Okay, I'll test the time consumed in memmap_init().
>>> But more generally, I wonder if we have to differentiate HOTPLUG here at all.
>>> @David, can you comment please?
>>>
>>>> +		}
>>>>    		/*
>>>>    		 * Usually, we want to mark the pageblock MIGRATE_MOVABLE,
>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>> index 06be8821d833..b868caabe8dc 100644
>>>> --- a/mm/page_alloc.c
>>>> +++ b/mm/page_alloc.c
>>>> @@ -1285,18 +1285,22 @@ void __free_pages_core(struct page *page, unsigned int order)
>>>>    	unsigned int loop;
>>>>    	/*
>>>> -	 * When initializing the memmap, __init_single_page() sets the refcount
>>>> -	 * of all pages to 1 ("allocated"/"not free"). We have to set the
>>>> -	 * refcount of all involved pages to 0.
>>>> +	 * When initializing the memmap, memmap_init_range sets the refcount
>>>> +	 * of all pages to 1 ("reserved" and "free") in hotplug context. We
>>>> +	 * have to set the refcount of all involved pages to 0. Otherwise,
>>>> +	 * we don't do it, as reserve_bootmem_region only set the refcount on
>>>> +	 * reserve region ("reserved") in early context.
>>>>    	 */
>>> Again, why hotplug and early init should be different?
>> I will add a comment that describes it will save boot time.
> But why do we need initialize struct pages differently at boot time vs
> memory hotplug?
> Is there a reason memory hotplug cannot have page count set to 0 just like
> for pages reserved at boot time?
>   

This patch just save boot time in MEMINIT_EARLY. If someone finds out 
that it can save time in

MEMINIT_HOTPLUG, I think it can be done in another patch later. I just 
keeping it in the same.

>>>> -	prefetchw(p);
>>>> -	for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
>>>> -		prefetchw(p + 1);
>>>> +	if (page_count(page)) {
>>>> +		prefetchw(p);
>>>> +		for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
>>>> +			prefetchw(p + 1);
>>>> +			__ClearPageReserved(p);
>>>> +			set_page_count(p, 0);
>>>> +		}
>>>>    		__ClearPageReserved(p);
>>>>    		set_page_count(p, 0);
>>>>    	}
>>>> -	__ClearPageReserved(p);
>>>> -	set_page_count(p, 0);
>>>>    	atomic_long_add(nr_pages, &page_zone(page)->managed_pages);
>>>> -- 
>>>> 2.25.1
>>>>