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

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Message-Id: <20210312072940.598696-1-leobras.c@gmail.com>
Date:   Fri, 12 Mar 2021 04:29:38 -0300
From:   Leonardo Bras <leobras.c@...il.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Leonardo Bras <leobras.c@...il.com>,
        Sandipan Das <sandipan@...ux.ibm.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Mike Rapoport <rppt@...nel.org>,
        Bharata B Rao <bharata@...ux.ibm.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Nathan Lynch <nathanl@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Scott Cheloha <cheloha@...ux.ibm.com>,
        David Gibson <david@...son.dropbear.id.au>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: [PATCH 0/3] powerpc/mm/hash: Time improvements for memory hot(un)plug

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