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Date: Mon, 12 Jun 2023 17:36:43 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: akpm@...ux-foundation.org, mgorman@...hsingularity.net,
vbabka@...e.cz, david@...hat.com, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: compaction: skip memory hole rapidly when isolating
migratable pages
On 6/12/2023 2:39 PM, Huang, Ying wrote:
> Baolin Wang <baolin.wang@...ux.alibaba.com> writes:
>
>> On some machines, the normal zone can have a large memory hole like
>> below memory layout, and we can see the range from 0x100000000 to
>> 0x1800000000 is a hole. So when isolating some migratable pages, the
>> scanner can meet the hole and it will take more time to skip the large
>> hole. From my measurement, I can see the isolation scanner will take
>> 80us ~ 100us to skip the large hole [0x100000000 - 0x1800000000].
>>
>> So adding a new helper to fast search next online memory section
>> to skip the large hole can help to find next suitable pageblock
>> efficiently. With this patch, I can see the large hole scanning only
>> takes < 1us.
>>
>> [ 0.000000] Zone ranges:
>> [ 0.000000] DMA [mem 0x0000000040000000-0x00000000ffffffff]
>> [ 0.000000] DMA32 empty
>> [ 0.000000] Normal [mem 0x0000000100000000-0x0000001fa7ffffff]
>> [ 0.000000] Movable zone start for each node
>> [ 0.000000] Early memory node ranges
>> [ 0.000000] node 0: [mem 0x0000000040000000-0x0000000fffffffff]
>> [ 0.000000] node 0: [mem 0x0000001800000000-0x0000001fa3c7ffff]
>> [ 0.000000] node 0: [mem 0x0000001fa3c80000-0x0000001fa3ffffff]
>> [ 0.000000] node 0: [mem 0x0000001fa4000000-0x0000001fa402ffff]
>> [ 0.000000] node 0: [mem 0x0000001fa4030000-0x0000001fa40effff]
>> [ 0.000000] node 0: [mem 0x0000001fa40f0000-0x0000001fa73cffff]
>> [ 0.000000] node 0: [mem 0x0000001fa73d0000-0x0000001fa745ffff]
>> [ 0.000000] node 0: [mem 0x0000001fa7460000-0x0000001fa746ffff]
>> [ 0.000000] node 0: [mem 0x0000001fa7470000-0x0000001fa758ffff]
>> [ 0.000000] node 0: [mem 0x0000001fa7590000-0x0000001fa7ffffff]
>>
>> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> ---
>> include/linux/mmzone.h | 10 ++++++++++
>> mm/compaction.c | 23 ++++++++++++++++++++++-
>> 2 files changed, 32 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index 5a7ada0413da..87e6c535d895 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -2000,6 +2000,16 @@ static inline unsigned long next_present_section_nr(unsigned long section_nr)
>> return -1;
>> }
>>
>> +static inline unsigned long next_online_section_nr(unsigned long section_nr)
>> +{
>> + while (++section_nr <= __highest_present_section_nr) {
>> + if (online_section_nr(section_nr))
>> + return section_nr;
>> + }
>> +
>> + return -1UL;
>> +}
>> +
>> /*
>> * These are _only_ used during initialisation, therefore they
>> * can use __initdata ... They could have names to indicate
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 3398ef3a55fe..3a55fdd20c49 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -229,6 +229,21 @@ static void reset_cached_positions(struct zone *zone)
>> pageblock_start_pfn(zone_end_pfn(zone) - 1);
>> }
>>
>> +static unsigned long skip_hole_pageblock(unsigned long start_pfn)
>> +{
>> + unsigned long next_online_nr;
>> + unsigned long start_nr = pfn_to_section_nr(start_pfn);
>> +
>> + if (online_section_nr(start_nr))
>> + return -1UL;
>
> Define a macro for the maigic "-1UL"? Which is used for multiple times
> in the patch.
I am struggling to find a readable macro for these '-1UL', since the
'-1UL' in next_online_section_nr() indicates that it can not find an
online section. However the '-1' in skip_hole_pageblock() indicates that
it can not find an online pfn.
So after more thinking, I will change to return 'NR_MEM_SECTIONS' if can
not find next online section in next_online_section_nr(). And in
skip_hole_pageblock(), I will change to return 0 if can not find next
online pfn. What do you think?
static unsigned long skip_hole_pageblock(unsigned long start_pfn)
{
unsigned long next_online_nr;
unsigned long start_nr = pfn_to_section_nr(start_pfn);
if (online_section_nr(start_nr))
return 0;
next_online_nr = next_online_section_nr(start_nr);
if (next_online_nr < NR_MEM_SECTIONS)
return section_nr_to_pfn(next_online_nr);
return 0;
}
>> +
>> + next_online_nr = next_online_section_nr(start_nr);
>> + if (next_online_nr != -1UL)
>> + return section_nr_to_pfn(next_online_nr);
>> +
>> + return -1UL;
>> +}
>> +
>> /*
>> * Compound pages of >= pageblock_order should consistently be skipped until
>> * released. It is always pointless to compact pages of such order (if they are
>> @@ -1991,8 +2006,14 @@ static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
>>
>> page = pageblock_pfn_to_page(block_start_pfn,
>> block_end_pfn, cc->zone);
>> - if (!page)
>> + if (!page) {
>> + unsigned long next_pfn;
>> +
>> + next_pfn = skip_hole_pageblock(block_start_pfn);
>> + if (next_pfn != -1UL)
>> + block_end_pfn = next_pfn;
>> continue;
>> + }
>>
>> /*
>> * If isolation recently failed, do not retry. Only check the
>
> Do we need to do similar change in isolate_freepages()?
Yes, it's in my todo list with some measurement data.
Thanks for your comments.
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