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

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Date:   Wed, 14 Nov 2018 17:55:12 -0800
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Cc:     Michal Hocko <mhocko@...nel.org>, 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 Wed, Nov 14, 2018 at 04:50:23PM -0800, Alexander Duyck wrote:
> 
> 
> 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.

Just recently we had a discussion about overlapping for_each_mem_range()
and for_each_mem_pfn_range(), but unfortunately it appears that no mailing
list was cc'ed by the original patch author :(
In short, there was a spelling fix in one of them and Michal pointed out
that their functionality overlaps.

I have no objection for for_each_free_mem_pfn_range_in_zone() and
__next_mem_pfn_range_in_zone(), but probably we should consider unifying
the older iterators before we introduce a new one? 
 
> >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.
> 
> 

-- 
Sincerely yours,
Mike.