Re: [PATCH v7 2/5] arm: arm64: page_alloc: reduce unnecessary binary search in memblock_next_valid_pfn()

From: Matthew Wilcox <willy@infradead.org>
To: Jia He <hejianet@gmail.com>
Cc: Russell King <linux@armlinux.org.uk>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@suse.com>,
	Wei Yang <richard.weiyang@gmail.com>,
	Kees Cook <keescook@chromium.org>,
	Laura Abbott <labbott@redhat.com>,
	Vladimir Murzin <vladimir.murzin@arm.com>,
	Philip Derrin <philip@cog.systems>,
	AKASHI Takahiro <takahiro.akashi@linaro.org>,
	James Morse <james.morse@arm.com>,
	Steve Capper <steve.capper@arm.com>,
	Pavel Tatashin <pasha.tatashin@oracle.com>,
	Gioh Kim <gi-oh.kim@profitbricks.com>,
	Vlastimil Babka <vbabka@suse.cz>, Mel Gorman <mgorman@suse.de>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Kemi Wang <kemi.wang@intel.com>, Petr Tesarik <ptesarik@suse.com>,
	YASUAKI ISHIMATSU <yasu.isimatu@gmail.com>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Nikolay Borisov <nborisov@suse.com>,
	Daniel Jordan <daniel.m.jordan@oracle.com>,
	Daniel Vacek <neelx@redhat.com>,
	Eugeniu Rosca <erosca@de.adit-jv.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Jia He <jia.he@hxt-semitech.com>
Subject: Re: [PATCH v7 2/5] arm: arm64: page_alloc: reduce unnecessary binary search in memblock_next_valid_pfn()
Date: Thu, 5 Apr 2018 05:50:54 -0700	[thread overview]
Message-ID: <20180405125054.GC2647@bombadil.infradead.org> (raw)
In-Reply-To: <1f809296-e88d-1090-0027-890782b91d6e@gmail.com>

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.