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From: Dan Williams <dan.j.williams@intel.com>
To: alexander.h.duyck@linux.intel.com
Cc: "Pasha Tatashin" <pavel.tatashin@microsoft.com>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	"Dave Hansen" <dave.hansen@intel.com>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Michal Hocko" <mhocko@kernel.org>,
	"Linux MM" <linux-mm@kvack.org>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	rppt@linux.vnet.ibm.com,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Ingo Molnar" <mingo@kernel.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: Wed, 26 Sep 2018 11:52:56 -0700	[thread overview]
Message-ID: <CAPcyv4iVnodai0bB74yeSCD2H+hoLsZYUk4sR9jV0pPAE+Zorw@mail.gmail.com> (raw)
In-Reply-To: <6f87a5d7-05e2-00f4-8568-bb3521869cea@linux.intel.com>

On Wed, Sep 26, 2018 at 11:25 AM Alexander Duyck
<alexander.h.duyck@linux.intel.com> wrote:
>
>
>
> On 9/26/2018 12:55 AM, Michal Hocko wrote:
> > On Tue 25-09-18 13:21:24, Alexander Duyck wrote:
> >> The ZONE_DEVICE pages were being initialized in two locations. One was with
> >> the memory_hotplug lock held and another was outside of that lock. The
> >> problem with this is that it was nearly doubling the memory initialization
> >> time. Instead of doing this twice, once while holding a global lock and
> >> once without, I am opting to defer the initialization to the one outside of
> >> the lock. This allows us to avoid serializing the overhead for memory init
> >> and we can instead focus on per-node init times.
> >>
> >> One issue I encountered is that devm_memremap_pages and
> >> hmm_devmmem_pages_create were initializing only the pgmap field the same
> >> way. One wasn't initializing hmm_data, and the other was initializing it to
> >> a poison value. Since this is something that is exposed to the driver in
> >> the case of hmm I am opting for a third option and just initializing
> >> hmm_data to 0 since this is going to be exposed to unknown third party
> >> drivers.
> >
> > Why cannot you pull move_pfn_range_to_zone out of the hotplug lock? In
> > other words why are you making zone device even more special in the
> > generic hotplug code when it already has its own means to initialize the
> > pfn range by calling move_pfn_range_to_zone. Not to mention the code
> > duplication.
>
> So there were a few things I wasn't sure we could pull outside of the
> hotplug lock. One specific example is the bits related to resizing the
> pgdat and zone. I wanted to avoid pulling those bits outside of the
> hotplug lock.
>
> The other bit that I left inside the hot-plug lock with this approach
> was the initialization of the pages that contain the vmemmap.
>
> > That being said I really dislike this patch.
>
> In my mind this was a patch that "killed two birds with one stone". I
> had two issues to address, the first one being the fact that we were
> performing the memmap_init_zone while holding the hotplug lock, and the
> other being the loop that was going through and initializing pgmap in
> the hmm and memremap calls essentially added another 20 seconds
> (measured for 3TB of memory per node) to the init time. With this patch
> I was able to cut my init time per node by that 20 seconds, and then
> made it so that we could scale as we added nodes as they could run in
> parallel.

Yeah, at the very least there is no reason for devm_memremap_pages()
to do another loop through all pages, the core should handle this, but
cleaning up the scope of the hotplug lock is needed.

> With that said I am open to suggestions if you still feel like I need to
> follow this up with some additional work. I just want to avoid
> introducing any regressions in regards to functionality or performance.

Could we push the hotplug lock deeper to the places that actually need
it? What I found with my initial investigation is that we don't even
need the hotplug lock for the vmemmap initialization with this patch
[1].

Alternatively it seems the hotplug lock wants to synchronize changes
to the zone and the page init work. If the hotplug lock was an rwsem
the zone changes would be a write lock, but the init work could be
done as a read lock to allow parallelism. I.e. still provide a sync
point to be able to assert that no hotplug work is in-flight will
holding the write lock, but otherwise allow threads that are touching
independent parts of the memmap to run at the same time.

[1]: https://patchwork.kernel.org/patch/10527229/ just focus on the
mm/sparse-vmemmap.c changes at the end.
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  reply	other threads:[~2018-09-26 18:53 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 [this message]
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
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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