linux-kernel - Re: [PATCH v9 0/6] optimize memblock_next_valid_pfn and early_pfn

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Date:   Mon, 2 Jul 2018 08:06:50 -0400
From:   Pavel Tatashin <pasha.tatashin@...cle.com>
To:     mhocko@...nel.org
Cc:     Jia He <hejianet@...il.com>, linux@...linux.org.uk,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mel Gorman <mgorman@...e.de>, will.deacon@....com,
        mark.rutland@....com, hpa@...or.com,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Gioh Kim <gi-oh.kim@...fitbricks.com>,
        Steven Sistare <steven.sistare@...cle.com>, neelx@...hat.com,
        erosca@...adit-jv.com, Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        james.morse@....com, Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        steve.capper@....com, tglx@...utronix.de, mingo@...hat.com,
        gregkh@...uxfoundation.org,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Johannes Weiner <hannes@...xchg.org>, kemi.wang@...el.com,
        Petr Tesařík <ptesarik@...e.com>,
        yasu.isimatu@...il.com, aryabinin@...tuozzo.com, nborisov@...e.com,
        Wei Yang <richard.weiyang@...il.com>
Subject: Re: [PATCH v9 0/6] optimize memblock_next_valid_pfn and
 early_pfn_valid on arm and arm64

On Mon, Jul 2, 2018 at 7:40 AM Michal Hocko <mhocko@...nel.org> wrote:
>
> On Fri 29-06-18 10:29:17, Jia He wrote:
> > Commit b92df1de5d28 ("mm: page_alloc: skip over regions of invalid pfns
> > where possible") tried to optimize the loop in memmap_init_zone(). But
> > there is still some room for improvement.
>
> It would be great to shortly describe those optimization from high level
> POV.
>
> >
> > Patch 1 introduce new config to make codes more generic
> > Patch 2 remain the memblock_next_valid_pfn on arm and arm64
> > Patch 3 optimizes the memblock_next_valid_pfn()
> > Patch 4~6 optimizes the early_pfn_valid()
> >
> > As for the performance improvement, after this set, I can see the time
> > overhead of memmap_init() is reduced from 27956us to 13537us in my
> > armv8a server(QDF2400 with 96G memory, pagesize 64k).
>
> So this is 13ms saving when booting 96G machine. Is this really worth
> the additional code? Are there any other benefits?

While 0.0144s for 96G is definitely small, I think the time is
proportional to the number of pages since memmap_init() loops through
all the pages. If base pages were changed to 4K, I bet the time would
increase 16 times: 0.23s on given machine, in other words around 2s
per 1T of memory.

I agree, a high level description of optimization is needed, and also
an explanation of why it would not work on other arches that support
memblock.

Pavel