From: Ralph Campbell <rcampbell@nvidia.com>
To: Jason Gunthorpe <jgg@mellanox.com>
Cc: "linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"nouveau@lists.freedesktop.org" <nouveau@lists.freedesktop.org>,
"linux-kselftest@vger.kernel.org"
<linux-kselftest@vger.kernel.org>,
Jerome Glisse <jglisse@redhat.com>,
"John Hubbard" <jhubbard@nvidia.com>,
Christoph Hellwig <hch@lst.de>,
Andrew Morton <akpm@linux-foundation.org>,
Ben Skeggs <bskeggs@redhat.com>, Shuah Khan <shuah@kernel.org>
Subject: Re: [PATCH v6 5/6] nouveau: use new mmu interval notifiers
Date: Thu, 16 Jan 2020 12:16:30 -0800 [thread overview]
Message-ID: <01adb7dd-589e-2cde-4fa9-68baa44c0976@nvidia.com> (raw)
In-Reply-To: <20200116160002.GL20978@mellanox.com>
On 1/16/20 8:00 AM, Jason Gunthorpe wrote:
> On Wed, Jan 15, 2020 at 02:09:47PM -0800, Ralph Campbell wrote:
>
>> I don't understand the lifetime/membership issue. The driver is the only thing
>> that allocates, inserts, or removes struct mmu_interval_notifier and thus
>> completely controls the lifetime.
>
> If the returned value is on the defered list it could be freed at any
> moment. The existing locks do not prevent it.
>
>>>> + ret = nouveau_svmm_interval_find(svmm, &range);
>>>> + if (ret) {
>>>> + up_read(&mm->mmap_sem);
>>>> + return ret;
>>>> + }
>>>> + range.notifier_seq = mmu_interval_read_begin(range.notifier);
>>>> ret = hmm_range_fault(&range, 0);
>>>> up_read(&mm->mmap_sem);
>>>> if (ret <= 0) {
>>>
>>> I'm still not sure this is a better approach than what ODP does. It
>>> looks very expensive on the fault path..
>>>
>>> Jason
>>>
>>
>> ODP doesn't have this problem because users have to call ib_reg_mr()
>> before any I/O can happen to the process address space.
>
> ODP supports a single 'full VA' call at process startup, just like
> these cases.
>
>> That is when mmu_interval_notifier_insert() /
>> mmu_interval_notifier_remove() can be called and the driver doesn't
>> have to worry about the interval changing sizes or being removed
>> while I/O is happening.
>
> No, for the 'ODP full process VA' (aka implicit ODP) mode it
> dynamically maintains a list of intervals. ODP chooses the align the
> dynamic intervals to it's HW page table levels, and not to SW VMAs.
> This is much simpler to manage and faster to fault, at the cost of
> capturing more VA for invalidations which have to be probed against
> the HW shadow PTEs.
>
>> It isn't that expensive, there is an extra driver lock/unlock as
>> part of the lookup and possibly a find_vma() and kmalloc(GFP_ATOMIC)
>> for new intervals. Also, the deferred interval updates for munmap().
>> Compared to the cost of updating PTEs in the device and GPU fault
>> handling, this is minimal overhead.
>
> Well, compared to ODP which does a single xa lookup with no lock to
> find its interval, this looks very expensive and not parallel.
>
> I think if there is merit in having ranges cover the vmas and track
> changes then there is probably merit in having the core code provide
> much of that logic, not the driver.
>
> But it would be interesting to see some kind of analysis on the two
> methods to decide if the complexity is worthwhile.
>
> Jason
>
Can you point me to the latest ODP code? Seems like my understanding is
quite off.
next prev parent reply other threads:[~2020-01-16 20:16 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-13 22:46 [PATCH v6 0/6] mm/hmm/test: add self tests for HMM Ralph Campbell
2020-01-13 22:46 ` [PATCH v6 1/6] mm/mmu_notifier: add mmu_interval_notifier_insert_safe() Ralph Campbell
2020-01-16 10:07 ` Christoph Hellwig
2020-01-13 22:46 ` [PATCH v6 2/6] mm/mmu_notifier: add mmu_interval_notifier_put() Ralph Campbell
2020-01-13 22:47 ` [PATCH v6 3/6] mm/notifier: add mmu_interval_notifier_update() Ralph Campbell
2020-01-13 22:47 ` [PATCH v6 4/6] mm/mmu_notifier: add mmu_interval_notifier_find() Ralph Campbell
2020-01-14 12:49 ` Jason Gunthorpe
2020-01-15 22:05 ` Ralph Campbell
2020-01-16 14:11 ` Jason Gunthorpe
2020-01-13 22:47 ` [PATCH v6 5/6] nouveau: use new mmu interval notifiers Ralph Campbell
2020-01-14 13:00 ` Jason Gunthorpe
2020-01-15 22:09 ` Ralph Campbell
2020-01-16 16:00 ` Jason Gunthorpe
2020-01-16 20:16 ` Ralph Campbell [this message]
2020-01-16 20:21 ` Jason Gunthorpe
2020-02-20 1:10 ` Ralph Campbell
2020-01-13 22:47 ` [PATCH v6 6/6] mm/hmm/test: add self tests for HMM Ralph Campbell
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=01adb7dd-589e-2cde-4fa9-68baa44c0976@nvidia.com \
--to=rcampbell@nvidia.com \
--cc=akpm@linux-foundation.org \
--cc=bskeggs@redhat.com \
--cc=hch@lst.de \
--cc=jgg@mellanox.com \
--cc=jglisse@redhat.com \
--cc=jhubbard@nvidia.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-kselftest@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=linux-rdma@vger.kernel.org \
--cc=nouveau@lists.freedesktop.org \
--cc=shuah@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).