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Date:   Tue, 13 Jun 2023 14:36:33 +0200
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Subject: Re: [PATCH v2] mm: compaction: skip memory hole rapidly when
 isolating migratable pages
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To:     Baolin Wang <baolin.wang@linux.alibaba.com>,
        akpm@linux-foundation.org
Cc:     mgorman@techsingularity.net, vbabka@suse.cz, ying.huang@intel.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <770f9f61472b24b6bc89adbd71a77d9cf62bb54f.1686646361.git.baolin.wang@linux.alibaba.com>
 <c9384e52-3f3b-cb1b-a607-955cd7066422@redhat.com>
 <838c3066-43e9-2035-cf69-4957481cddda@linux.alibaba.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <838c3066-43e9-2035-cf69-4957481cddda@linux.alibaba.com>
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On 13.06.23 13:13, Baolin Wang wrote:
> 
> 
> On 6/13/2023 5:56 PM, David Hildenbrand wrote:
>> On 13.06.23 10:55, Baolin Wang wrote:
>>> 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@linux.alibaba.com>
>>> ---
>>> Changes from v1:
>>>    - Fix building errors if CONFIG_SPARSEMEM is not selected.
>>>    - Use NR_MEM_SECTIONS instead of '-1' per Huang Ying.
>>> ---
>>>    include/linux/mmzone.h | 10 ++++++++++
>>>    mm/compaction.c        | 30 +++++++++++++++++++++++++++++-
>>>    2 files changed, 39 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>>> index 5a7ada0413da..5ff1fa2efe28 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 NR_MEM_SECTIONS;
>>> +}
>>> +
>>>    /*
>>>     * 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..c31ff6123891 100644
>>> --- a/mm/compaction.c
>>> +++ b/mm/compaction.c
>>> @@ -229,6 +229,28 @@ static void reset_cached_positions(struct zone
>>> *zone)
>>>                    pageblock_start_pfn(zone_end_pfn(zone) - 1);
>>>    }
>>> +#ifdef CONFIG_SPARSEMEM
>>> +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);
>>> +
>>
>> I would simply inline next_online_section_nr and simplify (and add a
>> comment):
>>
>> /*
>>    * If the PFN falls into an offline section, return the start PFN of the
>>    * next online section. If the PFN falls into an online section or if
>>    * there is no next online section, return 0.
>>    */
>> static unsigned long skip_hole_pageblock(unsigned long start_pfn)
>> {
>>       unsigned long nr = pfn_to_section_nr(start_pfn);
>>
>>       if (online_section_nr(nr))
>>           return 0;
>>
>>       while (++nr <= __highest_present_section_nr) {
>>           if (online_section_nr(nr))
>>               return section_nr_to_pfn(nr);
>>       }
>>       return 0
>> }
>>
>> Easier, no?
> 
> Originally I want to add a common helper like next_present_section_nr(),
> which can be used by other users. But yes, your suggestion is easier,
> and I am fine with that.
> 
>> And maybe just call that function "skip_offline_sections()" then?
>> Because we're not operating on pageblocks.
> 
> OK. Thanks.
> 

Feel free to add to the simplified version

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb