linux-kernel - Re: [PATCH 1/3] powerpc/mm/hash: Avoid resizing-down HPT on first memory hotplug

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Message-ID: <09c57b599f8dd95f4ff1c02b83aa093615257f2f.camel@gmail.com>
Date:   Thu, 08 Apr 2021 23:16:41 -0300
From:   Leonardo Bras <leobras.c@...il.com>
To:     David Gibson <david@...son.dropbear.id.au>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] powerpc/mm/hash: Avoid resizing-down HPT on first
 memory hotplug

Hello David, thanks for your feedback.

On Mon, 2021-03-22 at 17:49 +1100, David Gibson wrote:
> I don't love this approach.  Adding the extra flag at this level seems
> a bit inelegant, and it means we're passing up an easy opportunity to
> reduce our resource footprint on the host.

I understand, but trying to reduce resource footprint in host, and
mostly failing is what causes hot-add and hot-remove to take so long.

> But... maybe we'll have to do it.  I'd like to see if we can get
> things to work well enough with just the "batching" to avoid multiple
> resize attempts first.

This batching is something I had thought a lot about.
Problem is that there are a lot of generic interfaces between memory
hotplug and actually resizing HPT. I tried a simpler approach in
patches 2 & 3, so I don't touch much stuff there.

Best regards,
Leonardo Bras