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

From: Leonardo Bras <[email-masked]>
Date: Fri, 12 Mar 2021 04:29:38 -0300

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!
I believe the code may not be very well placed in available files,
so please give some feedback on that.

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/include/asm/sparsemem.h          |  4 +
 arch/powerpc/mm/book3s64/hash_utils.c         | 78 +++++++++++++++----
 arch/powerpc/mm/book3s64/pgtable.c            | 18 +++++
 .../platforms/pseries/hotplug-memory.c        | 22 ++++++
 5 files changed, 111 insertions(+), 15 deletions(-)

-- 
2.29.2