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

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Date:   Wed, 14 Nov 2018 16:50:23 -0800
From:   Alexander Duyck <alexander.h.duyck@...ux.intel.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     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 11/14/2018 7:07 AM, 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.

Actually it isn't so much that hotplug is special. The issue is more 
that the non-hotplug case is special in that you have to perform a 
number of extra checks for things that just aren't necessary for the 
hotplug case.

If anything I would probably need a new iterator that would be able to 
take into account all the checks for the non-hotplug case and then 
provide ranges of PFNs to initialize.

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

Yeah, the iterators were mostly an attempt at hiding some of the 
complexity. Being able to break a loop down to just an iterator provding 
the start of the range and the number of elements to initialize is 
pretty easy to visualize, or at least I thought so.

> Thing I do not like is that you keep microptimizing PageReserved part
> while there shouldn't be anything fundamental about it. We should just
> remove it rather than make the code more complex. I fell more and more
> guilty to add there actually.

I plan to remove it, but don't think I can get to it in this patch set.

I was planning to submit one more iteration of this patch set early next 
week, and then start focusing more on the removal of the PageReserved 
bit for hotplug. I figure it is probably going to be a full patch set 
onto itself and as you pointed out at the start of this email there is 
already enough code to review without adding that.