linux-kernel - Re: [PATCH 1/4] mm: page_alloc: reduce unnecessary binary search in memblock_next_valid

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Date:   Thu, 22 Mar 2018 13:52:04 +0100
From:   Eugeniu Rosca <erosca@...adit-jv.com>
To:     Jia He <hejianet@...il.com>
CC:     Daniel Vacek <neelx@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mel Gorman <mgorman@...e.de>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Pavel Tatashin <pasha.tatashin@...cle.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>,
        Vlastimil Babka <vbabka@...e.cz>,
        open list <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        James Morse <james.morse@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Steve Capper <steve.capper@....com>, <x86@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Kemi Wang <kemi.wang@...el.com>,
        Petr Tesarik <ptesarik@...e.com>,
        YASUAKI ISHIMATSU <yasu.isimatu@...il.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Nikolay Borisov <nborisov@...e.com>,
        Jia He <jia.he@...-semitech.com>,
        Eugeniu Rosca <erosca@...adit-jv.com>
Subject: Re: [PATCH 1/4] mm: page_alloc: reduce unnecessary binary search in
 memblock_next_valid_pfn()

On Wed, Mar 21, 2018 at 08:28:18PM +0800, Jia He wrote:
> 
> 
> On 3/21/2018 6:14 PM, Daniel Vacek Wrote:
> >On Wed, Mar 21, 2018 at 9:09 AM, Jia He <hejianet@...il.com> wrote:
> >>Commit b92df1de5d28 ("mm: page_alloc: skip over regions of invalid pfns
> >>where possible") optimized the loop in memmap_init_zone(). But there is
> >>still some room for improvement. E.g. if pfn and pfn+1 are in the same
> >>memblock region, we can simply pfn++ instead of doing the binary search
> >>in memblock_next_valid_pfn.
> >There is a revert-mm-page_alloc-skip-over-regions-of-invalid-pfns-where-possible.patch
> >in -mm reverting b92df1de5d289c0b as it is fundamentally wrong by
> >design causing system panics on some machines with rare but still
> >valid mappings. Basically it skips valid pfns which are outside of
> >usable memory ranges (outside of memblock memory regions).
> Thanks for the infomation.
> quote from you patch description:
> >But given some specific memory mapping on x86_64 (or more generally
> theoretically anywhere but on arm with CONFIG_HAVE_ARCH_PFN_VALID) > the
> implementation also skips valid pfns which is plain wrong and causes >
> 'kernel BUG at mm/page_alloc.c:1389!'
> 
> Do you think memblock_next_valid_pfn can remain to be not reverted on arm64
> with CONFIG_HAVE_ARCH_PFN_VALID? Arm64 can benifit from this optimization.

I confirm that the boot time of Rcar-H3 arm64 platform greatly
benefits from v4.11-rc1 commit b92df1de5d28 ("mm: page_alloc: skip over
regions of invalid pfns where possible"). The startup improvement is
roughly ~140ms, which will be lost if the mentioned commit is reverted.

For more details on my measurements, please see linux-next commit
283f1645e236 ("mm: page_alloc: skip over regions of invalid pfns on
UMA").

Whichever way you decide to go forward (reimplement/fix b92df1de5d28
or create an <arch>_next_valid_pfn), I am willing to participate in
testing your proposals on RCAR SoCs. TIA.

Thanks,
Eugeniu.