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Subject: Re: [PATCH v7 2/5] arm: arm64: page_alloc: reduce unnecessary binary
 search in memblock_next_valid_pfn()
To:     Russell King - ARM Linux <linux@armlinux.org.uk>,
        Matthew Wilcox <willy@infradead.org>
Cc:     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>
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 <20180406090920.GM16141@n2100.armlinux.org.uk>
From:   Jia He <hejianet@gmail.com>
Message-ID: <3308d7f2-e793-8f8e-7811-448b808bf736@gmail.com>
Date:   Sun, 8 Apr 2018 10:05:08 +0800
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Thanks for your comments, Russell


On 4/6/2018 5:09 PM, Russell King - ARM Linux Wrote:
> On Thu, Apr 05, 2018 at 05:50:54AM -0700, Matthew Wilcox wrote:
>> 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.
> That's been brought up before, and the reasoning appears to be
> something along the lines of...
>
> Academics and published wisdom is that on cached architectures, binary
> searches are bad because it doesn't operate efficiently due to the
> overhead from having to load cache lines.  Consequently, there seems
> to be a knee-jerk reaction that "all binary searches are bad, we must
> eliminate them."
IIUC, are you opposed to entirely removing the binary search instead of my
previous patch set?
>
> What is failed to be grasped here, though, is that it is typical that
> the number of entries in this array tend to be small, so the entire
> array takes up one or two cache lines, maybe a maximum of four lines
> depending on your cache line length and number of entries.
>
> This means that the binary search expense is reduced, and is lower
> than a linear search for the majority of cases.
>
> What is key here as far as performance is concerned is whether the
> general usage of pfn_valid() by the kernel is optimal.  We should
> not optimise only for the boot case, which means evaluating the
> effect of these changes with _real_ workloads, not just "does my
> machine boot a milliseconds faster".
hmm.. But pfn is linearly increased during the booting time. This assumption
is not correct in real workload for pfn_valid out of booting time. So in my
patchset, I defined another pfn_valid_region for booting time only.

I didn't have many arm/arm64 boxes to verifed. What I can do is guaranteeing
the improvemnet in my armv8a (qualcom centriq 2400). Sorry about it.

  --
Cheers,
Jia