linux-kernel - Re: [mm PATCH v5 0/7] Deferred page init improvements

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Date:   Wed, 14 Nov 2018 19:12:53 +0000
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvdimm@...ts.01.org, davem@...emloft.net,
        pavel.tatashin@...rosoft.com, mingo@...nel.org,
        kirill.shutemov@...ux.intel.com, dan.j.williams@...el.com,
        dave.jiang@...el.com, rppt@...ux.vnet.ibm.com, willy@...radead.org,
        vbabka@...e.cz, khalid.aziz@...cle.com, ldufour@...ux.vnet.ibm.com,
        mgorman@...hsingularity.net, yi.z.zhang@...ux.intel.com
Subject: Re: [mm PATCH v5 0/7] Deferred page init improvements

On 18-11-14 16:07:42, Michal Hocko wrote:
> On Mon 05-11-18 13:19:25, Alexander Duyck wrote:
> > This patchset is essentially a refactor of the page initialization logic
> > that is meant to provide for better code reuse while providing a
> > significant improvement in deferred page initialization performance.
> > 
> > In my testing on an x86_64 system with 384GB of RAM and 3TB of persistent
> > memory per node I have seen the following. In the case of regular memory
> > initialization the deferred init time was decreased from 3.75s to 1.06s on
> > average. For the persistent memory the initialization time dropped from
> > 24.17s to 19.12s on average. This amounts to a 253% improvement for the
> > deferred memory initialization performance, and a 26% improvement in the
> > persistent memory initialization performance.
> > 
> > I have called out the improvement observed with each patch.
> 
> I have only glanced through the code (there is a lot of the code to look
> at here). And I do not like the code duplication and the way how you
> make the hotplug special. There shouldn't be any real reason for that
> IMHO (e.g. why do we init pfn-at-a-time in early init while we do
> pageblock-at-a-time for hotplug). I might be wrong here and the code
> reuse might be really hard to achieve though.

I do not like having __init_single_page() to be done differently for
hotplug. I think, if that is fixed, there is almost no more code
duplication, the rest looks alright to me.

> 
> I am also not impressed by new iterators because this api is quite
> complex already. But this is mostly a detail.

I have reviewed all the patches in this series, and at first was also
worried about the new iterators. But, after diving deeper, they actually
make sense, and new memblock iterators look alright to me. The only
iterator, that I would like to see improved is
for_each_deferred_pfn_valid_range(), because it is very hard to
understand it.

This series is an impressive performance improvement, and I have
confirmed it on both arm, and x86. It is also should be compatible with
ktasks.

> 
> Thing I do not like is that you keep microptimizing PageReserved part
> while there shouldn't be anything fundamental about it. We should just

Agree.

> remove it rather than make the code more complex. I fell more and more
> guilty to add there actually.