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Date:   Sun, 1 Oct 2023 21:59:34 +0300
From:   Mike Rapoport <rppt@kernel.org>
To:     Yajun Deng <yajun.deng@linux.dev>
Cc:     akpm@linux-foundation.org, mike.kravetz@oracle.com,
        muchun.song@linux.dev, willy@infradead.org, david@redhat.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region
 when MEMINIT_EARLY
Message-ID: <20231001185934.GX3303@kernel.org>
References: <20230928083302.386202-1-yajun.deng@linux.dev>
 <20230928083302.386202-3-yajun.deng@linux.dev>
 <20230929083018.GU3303@kernel.org>
 <a6a20ff9-385c-639f-75cf-ce73a01d97cf@linux.dev>
 <20230929100252.GW3303@kernel.org>
 <15233624-f32e-172e-b2f6-7ca7bffbc96d@linux.dev>
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In-Reply-To: <15233624-f32e-172e-b2f6-7ca7bffbc96d@linux.dev>
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On Fri, Sep 29, 2023 at 06:27:25PM +0800, Yajun Deng wrote:
> 
> 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.

But it's not the same. It becomes slower after your patch and the code that
frees the pages for MEMINIT_EARLY and MEMINIT_HOTPLUG becomes non-uniform
for no apparent reason.

The if (page_count(page)) is really non-obvious...
 
> > > > > -	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
> > > > > 

-- 
Sincerely yours,
Mike.