linux-kernel - Re: [PATCH v2 1/3] powerpc/mm/hash: Avoid resizing-down HPT on first memory hotplug

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Date:   Tue, 08 Jun 2021 21:52:10 -0300
From:   Leonardo Brás <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>,
        Sandipan Das <sandipan@...ux.ibm.com>,
        Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Nathan Lynch <nathanl@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Scott Cheloha <cheloha@...ux.ibm.com>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] powerpc/mm/hash: Avoid resizing-down HPT on
 first memory hotplug

On Mon, 2021-06-07 at 15:02 +1000, David Gibson wrote:
> On Fri, Apr 30, 2021 at 11:36:06AM -0300, Leonardo Bras wrote:
> > Because hypervisors may need to create HPTs without knowing the
> > guest
> > page size, the smallest used page-size (4k) may be chosen,
> > resulting in
> > a HPT that is possibly bigger than needed.
> > 
> > On a guest with bigger page-sizes, the amount of entries for HTP
> > may be
> > too high, causing the guest to ask for a HPT resize-down on the
> > first
> > hotplug.
> > 
> > This becomes a problem when HPT resize-down fails, and causes the
> > HPT resize to be performed on every LMB added, until HPT size is
> > compatible to guest memory size, causing a major slowdown.
> > 
> > So, avoiding HPT resizing-down on hot-add significantly improves
> > memory
> > hotplug times.
> > 
> > As an example, hotplugging 256GB on a 129GB guest took 710s without
> > this
> > patch, and 21s after applied.
> > 
> > Signed-off-by: Leonardo Bras <leobras.c@...il.com>
> 
> Sorry it's taken me so long to look at these
> 
> I don't love the extra statefulness that the 'shrinking' parameter
> adds, but I can't see an elegant way to avoid it, so:
> 
> Reviewed-by: David Gibson <david@...son.dropbear.id.au>

np, thanks for reviewing!

Best regards,
Leonardo Bras