alloc_contig_range() with MIGRATE_MOVABLE performance regression since 4.9

[Florian Fainelli <f.fainelli@gmail.com>]

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Hi all,

I have been trying for the past few days to identify the source of a
performance regression that we are seeing with the 5.4 kernel but not
with the 4.9 kernel on ARM64. Testing something newer like 5.10 is a bit
challenging at the moment but will happen eventually.

What we are seeing is a ~3x increase in the time needed for
alloc_contig_range() to allocate 1GB in blocks of 2MB pages. The system
is idle at the time and there are no other contenders for memory other
than the user-space programs already started (DHCP client, shell, etc.).

I have tried playing with the compact_control structure settings but
have not found anything that would bring us back to the performance of
4.9. More often than not, we see test_pages_isolated() returning an
non-zero error code which would explain the slow down, since we have
some logic that re-tries the allocation if alloc_contig_range() returns
-EBUSY. If I remove the retry logic however, we don't get -EBUSY and we
get the results below:

4.9 shows this:

[  457.537634] allocating: size: 1024MB avg: 59172 (us), max: 137306
(us), min: 44859 (us), total: 591723 (us), pages: 512, per-page: 115 (us)
[  457.550222] freeing: size: 1024MB avg: 67397 (us), max: 151408 (us),
min: 52630 (us), total: 673974 (us), pages: 512, per-page: 131 (us)

5.4 show this:

[  222.388758] allocating: size: 1024MB avg: 156739 (us), max: 157254
(us), min: 155915 (us), total: 1567394 (us), pages: 512, per-page: 306 (us)
[  222.401601] freeing: size: 1024MB avg: 209899 (us), max: 210085 (us),
min: 209749 (us), total: 2098999 (us), pages: 512, per-page: 409 (us)

This regression is not seen when MIGRATE_CMA is specified instead of
MIGRATE_MOVABLE.

A few characteristics that you should probably be aware of:

- There is 4GB of memory populated with the memory being mapped into the
CPU's address starting at space at 0x4000_0000 (1GB), PAGE_SIZE is 4KB

- there is a ZONE_DMA32 that starts at 0x4000_0000 and ends at
0xE480_0000, from there on we have a ZONE_MOVABLE which is comprised of
0xE480_0000 - 0xfdc00000 and another range spanning 0x1_0000_0000 -
0x1_4000_0000

Attached is the kernel configuration.

Thanks!
-- 
Florian

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