[PATCH v2 0/3] powerpc/mm/hash: Time improvements for memory hot(un)plug

Leonardo Bras leobras.c at gmail.com
Sat May 1 00:36:04 AEST 2021

This patchset intends to reduce time needed for processing memory
hotplug/hotunplug in hash guests.

The first one, makes sure guests with pagesize over 4k don't need to
go through HPT resize-downs after memory hotplug.

The second and third patches make hotplug / hotunplug perform a single
HPT resize per operation, instead of one for each shift change, or one
for each LMB in case of resize-down error.

Why haven't the same mechanism used for both memory hotplug and hotunplug?
They both have different requirements:

Memory hotplug causes (usually) HPT resize-ups, which are fine happening
at the start of hotplug, but resize-ups should not ever be disabled, as
other mechanisms may try to increase memory, hitting issues with a HPT
that is too small.

Memory hotunplug causes HPT resize-downs, which can be disabled (HPT will
just remain larger for a while), but need to happen at the end of an
hotunplug operation. If we want to batch it, we need to disable
resize-downs and perform it only at the end.

Tests done with this patchset in the same machine / guest config:
Starting memory: 129GB, DIMM: 256GB
Before patchset: hotplug = 710s, hotunplug = 621s.
After patchset: hotplug  = 21s, hotunplug = 100s.

Any feedback will be appreciated!

Changes since v1:
- Atomic used to disable resize was replaced by a mutex
- Removed wrappers, testing for !radix directly in hot(un)plug routine
- Added bounds to HPT resize loop
- Removed batching from dlpar_memory_*_by_index, as it adds a single LMB 

Best regards,
Leonardo Bras (3):
  powerpc/mm/hash: Avoid resizing-down HPT on first memory hotplug
  powerpc/mm/hash: Avoid multiple HPT resize-ups on memory hotplug
  powerpc/mm/hash: Avoid multiple HPT resize-downs on memory hotunplug

 arch/powerpc/include/asm/book3s/64/hash.h     |  4 +
 arch/powerpc/mm/book3s64/hash_utils.c         | 95 ++++++++++++++++---
 .../platforms/pseries/hotplug-memory.c        | 35 +++++++
 3 files changed, 119 insertions(+), 15 deletions(-)


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