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From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: "Pasha Tatashin" <pavel.tatashin@microsoft.com>,
	osalvador@techadventures.net,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	"Dave Hansen" <dave.hansen@intel.com>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Ingo Molnar" <mingo@kernel.org>, "Linux MM" <linux-mm@kvack.org>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	rppt@linux.vnet.ibm.com,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap
Date: Mon, 29 Oct 2018 12:59:11 -0700	[thread overview]
Message-ID: <3281f3044fa231bbc1b02d5c5efca3502a0d05a8.camel@linux.intel.com> (raw)
In-Reply-To: <20181029181827.GO32673@dhcp22.suse.cz>

On Mon, 2018-10-29 at 19:18 +0100, Michal Hocko wrote:
> On Mon 29-10-18 10:42:33, Alexander Duyck wrote:
> > On Mon, 2018-10-29 at 18:24 +0100, Michal Hocko wrote:
> > > On Mon 29-10-18 10:01:28, Alexander Duyck wrote:
> 
> [...]
> > > > So there end up being a few different issues with constructors. First
> > > > in my mind is that it means we have to initialize the region of memory
> > > > and cannot assume what the constructors are going to do for us. As a
> > > > result we will have to initialize the LRU pointers, and then overwrite
> > > > them with the pgmap and hmm_data.
> > > 
> > > Why we would do that? What does really prevent you from making a fully
> > > customized constructor?
> > 
> > It is more an argument of complexity. Do I just pass a single pointer
> > and write that value, or the LRU values in init, or do I have to pass a
> > function pointer, some abstracted data, and then call said function
> > pointer while passing the page and the abstracted data?
> 
> I though you have said that pgmap is the current common denominator for
> zone device users. I really do not see what is the problem to do
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 89d2a2ab3fe6..9105a4ed2c96 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5516,7 +5516,10 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>  
>  not_early:
>  		page = pfn_to_page(pfn);
> -		__init_single_page(page, pfn, zone, nid);
> +		if (pgmap && pgmap->init_page)
> +			pgmap->init_page(page, pfn, zone, nid, pgmap);
> +		else
> +			__init_single_page(page, pfn, zone, nid);
>  		if (context == MEMMAP_HOTPLUG)
>  			SetPageReserved(page);
>  
> that would require to replace altmap throughout the call chain and
> replace it by pgmap. Altmap could be then renamed to something more
> clear

So as I had pointed out earlier doing per-page init is much slower than
initializing pages in bulk. Ideally I would like to see us seperate the
memmap_init_zone function into two pieces, one section for handling
hotplug and another for the everything else case. As is the fact that
you have to jump over a bunch of tests for the "not_early" case is
quite ugly in my opinion.

I could probably take your patch and test it. I'm suspecting this is
going to be a signficant slow-down in general as the indirect function
pointer stuff is probably going to come into play.

The "init_page" function in this case is going to end up being much
more complex then it really needs to be in this design as well since I
have to get the altmap and figure out if the page was used for vmmemmap
storage or is an actual DAX page. I might just see if I could add an
additional test for the pfn being before the end of the vmmemmap in the
case of pgmap being present.

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 89d2a2ab3fe6..048e4cc72fdf 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5474,8 +5474,8 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>  	 * Honor reservation requested by the driver for this ZONE_DEVICE
>  	 * memory
>  	 */
> -	if (altmap && start_pfn == altmap->base_pfn)
> -		start_pfn += altmap->reserve;
> +	if (pgmap && pgmap->get_memmap)
> +		start_pfn = pgmap->get_memmap(pgmap, start_pfn);
>  
>  	for (pfn = start_pfn; pfn < end_pfn; pfn++) {
>  		/*
> 
> [...]

The only reason why I hadn't bothered with these bits is that I was
actually trying to leave this generic since I thought I had seen other
discussions about hotplug scenerios where memory may want to change
where the vmmemmap is initialized other than just the case of
ZONE_DEVICE pages. So I was thinking at some point we may see altmap
without the pgmap.

> > If I have to implement the code to verify the slowdown I will, but I
> > really feel like it is just going to be time wasted since we have seen
> > this in other spots within the kernel.
> 
> Please try to understand that I am not trying to force you write some
> artificial benchmarks. All I really do care about is that we have sane
> interfaces with reasonable performance. Especially for one-off things
> in relattively slow paths. I fully recognize that ZONE_DEVICE begs for a
> better integration but really, try to go incremental and try to unify
> the code first and microptimize on top. Is that way too much to ask for?

No, but the patches I had already proposed I thought were heading in
that direction. I had unified memmap_init_zone,
memmap_init_zone_device, and the deferred page initialization onto a
small set of functions and all had improved performance as a result.

> Anyway we have gone into details while the primary problem here was that
> the hotplug lock doesn't scale AFAIR. And my question was why cannot we
> pull move_pfn_range_to_zone and what has to be done to achieve that.
> That is a fundamental thing to address first. Then you can microptimize
> on top.

Yes, the hotplug lock was part of the original issue. However that
starts to drift into the area I believe Oscar was working on as a part
of his patch set in encapsulating the move_pfn_range_to_zone and other
calls that were contained in the hotplug lock into their own functions.

Most of the changes I have in my follow-on patch set can work
regardless of how we deal with the lock issue. I just feel like what
you are pushing for is going to be a massive patch set by the time we
are done and I really need to be able to work this a piece at a time. 

The patches Andrew pushed addressed the immediate issue so that now
systems with nvdimm/DAX memory can at least initialize quick enough
that systemd doesn't refuse to mount the root file system due to a
timeout. The next patch set I have refactors things to reduce code and
allow us to reuse some of the hotplug code for the deferred page init, 
https://lore.kernel.org/lkml/20181017235043.17213.92459.stgit@localhost.localdomain/
. After that I was planning to work on dealing with the PageReserved
flag and trying to get that sorted out.

I was hoping to wait until after Dan's HMM patches and Oscar's changes
had been sorted before I get into any further refactor of this specific
code.

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  reply	other threads:[~2018-10-29 19:59 UTC|newest]

Thread overview: 77+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-25 20:18 [PATCH v5 0/4] Address issues slowing persistent memory initialization Alexander Duyck
2018-09-25 20:19 ` [PATCH v5 1/4] mm: Remove now defunct NO_BOOTMEM from depends list for deferred init Alexander Duyck
2018-09-25 21:05   ` Mike Rapoport
2018-09-25 20:20 ` [PATCH v5 2/4] mm: Provide kernel parameter to allow disabling page init poisoning Alexander Duyck
2018-09-25 20:26   ` Dave Hansen
2018-09-25 20:38     ` Alexander Duyck
2018-09-25 22:14       ` Dave Hansen
2018-09-25 22:27         ` Alexander Duyck
2018-09-26  7:38   ` Michal Hocko
2018-09-26 15:24     ` Alexander Duyck
2018-09-26 15:39       ` Michal Hocko
2018-09-26 15:41       ` Dave Hansen
2018-09-26 16:18         ` Alexander Duyck
2018-09-26 15:36     ` Dave Hansen
2018-09-26 22:36       ` Andrew Morton
2018-09-25 20:20 ` [PATCH v5 3/4] mm: Create non-atomic version of SetPageReserved for init use Alexander Duyck
2018-09-25 20:21 ` [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap Alexander Duyck
2018-09-26  7:55   ` Michal Hocko
2018-09-26 18:25     ` Alexander Duyck
2018-09-26 18:52       ` Dan Williams
2018-09-27 11:20         ` Michal Hocko
2018-09-27 11:09       ` Michal Hocko
2018-09-27 12:25         ` Oscar Salvador
2018-09-27 13:13           ` Michal Hocko
2018-09-27 14:50             ` Oscar Salvador
2018-09-27 15:41               ` David Hildenbrand
2018-09-28  8:12             ` Oscar Salvador
2018-09-28  8:44               ` Oscar Salvador
2018-09-28 15:50                 ` Dan Williams
2018-09-27 12:32       ` Oscar Salvador
2018-10-08 21:01   ` Dan Williams
2018-10-08 21:38     ` Alexander Duyck
2018-10-08 22:00       ` Dan Williams
2018-10-08 22:07         ` Alexander Duyck
2018-10-08 22:36         ` Alexander Duyck
2018-10-08 22:59           ` Dan Williams
2018-10-08 23:34     ` [mm PATCH] memremap: Fix reference count for pgmap in devm_memremap_pages Alexander Duyck
2018-10-09  0:20       ` Dan Williams
2018-10-09 17:00   ` [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap Yi Zhang
2018-10-09 18:04     ` Dan Williams
2018-10-09 20:26       ` Alexander Duyck
2018-10-09 21:19         ` Dan Williams
2018-10-10 12:52           ` Yi Zhang
2018-10-10 15:27             ` Alexander Duyck
2018-10-11  8:17               ` Yi Zhang
2018-10-10  9:58         ` Michal Hocko
2018-10-10 16:39           ` Alexander Duyck
2018-10-10 17:24             ` Michal Hocko
2018-10-10 17:39               ` Alexander Duyck
2018-10-10 17:53                 ` Michal Hocko
2018-10-10 18:13                   ` Alexander Duyck
2018-10-10 18:52                 ` Michal Hocko
2018-10-11  8:55                   ` Michal Hocko
2018-10-11 17:38                     ` Alexander Duyck
2018-10-11 18:22                       ` Dan Williams
2018-10-17  7:52                       ` Michal Hocko
2018-10-17 15:02                         ` Alexander Duyck
2018-10-29 14:12                           ` Michal Hocko
2018-10-29 15:59                             ` Alexander Duyck
2018-10-29 16:35                               ` Michal Hocko
2018-10-29 17:01                                 ` Alexander Duyck
2018-10-29 17:24                                   ` Michal Hocko
2018-10-29 17:34                                     ` Dan Williams
2018-10-29 17:45                                       ` Michal Hocko
2018-10-29 17:42                                     ` Alexander Duyck
2018-10-29 18:18                                       ` Michal Hocko
2018-10-29 19:59                                         ` Alexander Duyck [this message]
2018-10-30  6:29                                           ` Michal Hocko
2018-10-30  6:55                                             ` Dan Williams
2018-10-30  8:17                                               ` Michal Hocko
2018-10-30 15:57                                                 ` Dan Williams
2018-10-30  8:05                                           ` Oscar Salvador
2018-10-29 15:49                           ` Dan Williams
2018-10-29 15:56                             ` Michal Hocko
2018-10-10 18:18               ` Dan Williams
2018-10-11  8:39                 ` Yi Zhang
2018-10-11 15:38                   ` Alexander Duyck

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