linux-kernel - Re: [PATCH v7 2/5] arm: arm64: page_alloc: reduce unnecessary binary search in memblock_next_valid

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Message-ID: <20180405125054.GC2647@bombadil.infradead.org>
Date:   Thu, 5 Apr 2018 05:50:54 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Jia He <hejianet@...il.com>
Cc:     Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Philip Derrin <philip@....systems>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        James Morse <james.morse@....com>,
        Steve Capper <steve.capper@....com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Gioh Kim <gi-oh.kim@...fitbricks.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mel Gorman <mgorman@...e.de>,
        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>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Daniel Vacek <neelx@...hat.com>,
        Eugeniu Rosca <erosca@...adit-jv.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Jia He <jia.he@...-semitech.com>
Subject: Re: [PATCH v7 2/5] arm: arm64: page_alloc: reduce unnecessary binary
 search in memblock_next_valid_pfn()

On Thu, Apr 05, 2018 at 08:44:12PM +0800, Jia He wrote:
> 
> 
> On 4/5/2018 7:34 PM, Matthew Wilcox Wrote:
> > On Thu, Apr 05, 2018 at 01:04:35AM -0700, Jia He 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.
> > Sure, but I bet if we are >end_pfn, we're almost certainly going to the
> > start_pfn of the next block, so why not test that as well?
> > 
> > > +	/* fast path, return pfn+1 if next pfn is in the same region */
> > > +	if (early_region_idx != -1) {
> > > +		start_pfn = PFN_DOWN(regions[early_region_idx].base);
> > > +		end_pfn = PFN_DOWN(regions[early_region_idx].base +
> > > +				regions[early_region_idx].size);
> > > +
> > > +		if (pfn >= start_pfn && pfn < end_pfn)
> > > +			return pfn;
> > 		early_region_idx++;
> > 		start_pfn = PFN_DOWN(regions[early_region_idx].base);
> > 		if (pfn >= end_pfn && pfn <= start_pfn)
> > 			return start_pfn;
> Thanks, thus the binary search in next step can be discarded?

I don't know all the circumstances in which this is called.  Maybe a linear
search with memo is more appropriate than a binary search.