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Date:   Fri, 14 Sep 2018 20:50:51 +0200
From:   Eugeniu Rosca <erosca@de.adit-jv.com>
To:     Jia He <hejianet@gmail.com>, Will Deacon <will.deacon@arm.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>
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        Mark Rutland <mark.rutland@arm.com>,
        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>,
        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>,
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        Linux-MM <linux-mm@kvack.org>, Jia He <jia.he@hxt-semitech.com>,
        "George G. Davis" <george_davis@mentor.com>,
        Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>,
        Andy Lowe <andy_lowe@mentor.com>,
        <linux-renesas-soc@vger.kernel.org>,
        Eugeniu Rosca <roscaeugeniu@gmail.com>,
        Eugeniu Rosca <erosca@de.adit-jv.com>
Subject: Re: [PATCH v11 0/3] remain and optimize memblock_next_valid_pfn on
 arm and arm64
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+ Renesas people

Hello Will, hello Ard, 

On Fri, Sep 07, 2018 at 03:44:47PM +0100, Will Deacon wrote:
> On Thu, Sep 06, 2018 at 01:24:22PM +0200, Ard Biesheuvel wrote:
> > OK so we can summarize the benefits of this series as follows:
> > - boot time on a virtual model of a Samurai CPU drops from 109 to 62 seconds
> > - boot time on a QDF2400 arm64 server with 96 GB of RAM drops by ~15
> > *milliseconds*
> > 
> > Google was not very helpful in figuring out what a Samurai CPU is and
> > why we should care about the boot time of Linux running on a virtual
> > model of it, and the 15 ms speedup is not that compelling either.
> > 
> > Apologies to Jia that it took 11 revisions to reach this conclusion,
> > but in /my/ opinion, tweaking the fragile memblock/pfn handling code
> > for this reason is totally unjustified, and we're better off
> > disregarding these patches.
> 
> Oh, we're talking about a *simulator* for the significant boot time
> improvement here? I didn't realise that, so I agree that the premise of
> this patch set looks pretty questionable given how much "fun" we've had
> with the memmap on arm and arm64.
> 
> Will

Similar to https://lkml.org/lkml/2018/1/24/420, my measurements show that
the boot time of R-Car H3-ES2.0 Salvator-X (having 4GiB RAM) is decreased
by ~135-140ms with this patch-set applied on top of v4.19-rc3.

I agree that in the Desktop realm you would barely perceive the 140ms
difference, but saving 140ms on the automotive SoC (designed for products
which must comply with 2s-to-rear-view-camera NHTSA US regulations) *is*
significant.

FWIW, cppcheck and `checkpatch --strict` report style issues for
patches #2 and #3. I hope these can be fixed and the review process
can go on? From functional standpoint, I did some dynamic testing on
H3-Salvator-X with UBSAN/KASAN=y and didn't observe any regressions, so:

Tested-by: Eugeniu Rosca <erosca@de.adit-jv.com>

Best regards,
Eugeniu.