Re: [PATCH v2] mm: compaction: skip memory hole rapidly when isolating migratable pages

["Huang, Ying" <ying.huang@intel.com>]

Hi, Mel,

Mel Gorman <mgorman@techsingularity.net> writes:

> On Tue, Jun 13, 2023 at 04:55:04PM +0800, 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>
>
> This may only be necessary for non-contiguous zones so a check for
> zone_contiguous could be made but I suspect the saving, if any, would be
> marginal.
>
> However, it's subtle that block_end_pfn can end up in an arbirary location
> past the end of the zone or past cc->free_pfn. As the "continue" will update
> cc->migrate_pfn, that might lead to errors in the future. It would be a
> lot safer to pass in cc->free_pfn and do two things with the value. First,
> there is no point scanning for a valid online section past cc->free_pfn so
> terminating after cc->free_pfn may save some cycles. Second, cc->migrate_pfn
> does not end up with an arbitrary value which is a more defensive approach
> to any future programming errors.

I have thought about this before.  Originally, I had thought that we
were safe because cc->free_pfn should be in a online section and
block_end_pfn should reach cc->free_pfn before the end of zone.  But
after checking more code and thinking about it again, I found that the
underlying sections may go offline under us during compaction.  So that,
cc->free_pfn may be in a offline section or after the end of zone.  So,
you are right, we need to consider the range of block_end_pfn.

But, if we thought in this way (memory online/offline at any time), it
appears that we need to check whether the underlying section was
offlined.  For example, is it safe to use "pfn_to_page()" in
"isolate_migratepages_block()"?  Is it possible for the underlying
section to be offlined under us?

Hi, David, can you teach me on this too?

Best Regards,
Huang, Ying