From: "Huang, Ying" <ying.huang@intel.com>
To: Jan Kara <jack@suse.cz>
Cc: Hugh Dickins <hughd@google.com>,
Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
linux-fsdevel@vger.kernel.org, Zi Yan <ziy@nvidia.com>,
Yang Shi <shy828301@gmail.com>,
Baolin Wang <baolin.wang@linux.alibaba.com>,
Oscar Salvador <osalvador@suse.de>,
Matthew Wilcox <willy@infradead.org>,
Bharata B Rao <bharata@amd.com>,
Alistair Popple <apopple@nvidia.com>,
Xin Hao <xhao@linux.alibaba.com>,
Minchan Kim <minchan@kernel.org>,
Mike Kravetz <mike.kravetz@oracle.com>,
Hyeonggon Yoo <42.hyeyoo@gmail.com>
Subject: Re: [PATCH -v5 0/9] migrate_pages(): batch TLB flushing
Date: Tue, 28 Feb 2023 09:13:26 +0800 [thread overview]
Message-ID: <87pm9ubnih.fsf@yhuang6-desk2.ccr.corp.intel.com> (raw)
In-Reply-To: <20230227110614.dngdub2j3exr6dfp@quack3> (Jan Kara's message of "Mon, 27 Feb 2023 12:06:14 +0100")
Hi, Honza,
Jan Kara <jack@suse.cz> writes:
> On Fri 17-02-23 13:47:48, Hugh Dickins wrote:
>> On Mon, 13 Feb 2023, Huang Ying wrote:
>>
>> > From: "Huang, Ying" <ying.huang@intel.com>
>> >
>> > Now, migrate_pages() migrate folios one by one, like the fake code as
>> > follows,
>> >
>> > for each folio
>> > unmap
>> > flush TLB
>> > copy
>> > restore map
>> >
>> > If multiple folios are passed to migrate_pages(), there are
>> > opportunities to batch the TLB flushing and copying. That is, we can
>> > change the code to something as follows,
>> >
>> > for each folio
>> > unmap
>> > for each folio
>> > flush TLB
>> > for each folio
>> > copy
>> > for each folio
>> > restore map
>> >
>> > The total number of TLB flushing IPI can be reduced considerably. And
>> > we may use some hardware accelerator such as DSA to accelerate the
>> > folio copying.
>> >
>> > So in this patch, we refactor the migrate_pages() implementation and
>> > implement the TLB flushing batching. Base on this, hardware
>> > accelerated folio copying can be implemented.
>> >
>> > If too many folios are passed to migrate_pages(), in the naive batched
>> > implementation, we may unmap too many folios at the same time. The
>> > possibility for a task to wait for the migrated folios to be mapped
>> > again increases. So the latency may be hurt. To deal with this
>> > issue, the max number of folios be unmapped in batch is restricted to
>> > no more than HPAGE_PMD_NR in the unit of page. That is, the influence
>> > is at the same level of THP migration.
>> >
>> > We use the following test to measure the performance impact of the
>> > patchset,
>> >
>> > On a 2-socket Intel server,
>> >
>> > - Run pmbench memory accessing benchmark
>> >
>> > - Run `migratepages` to migrate pages of pmbench between node 0 and
>> > node 1 back and forth.
>> >
>> > With the patch, the TLB flushing IPI reduces 99.1% during the test and
>> > the number of pages migrated successfully per second increases 291.7%.
>> >
>> > Xin Hao helped to test the patchset on an ARM64 server with 128 cores,
>> > 2 NUMA nodes. Test results show that the page migration performance
>> > increases up to 78%.
>> >
>> > This patchset is based on mm-unstable 2023-02-10.
>>
>> And back in linux-next this week: I tried next-20230217 overnight.
>>
>> There is a deadlock in this patchset (and in previous versions: sorry
>> it's taken me so long to report), but I think one that's easily solved.
>>
>> I've not bisected to precisely which patch (load can take several hours
>> to hit the deadlock), but it doesn't really matter, and I expect that
>> you can guess.
>>
>> My root and home filesystems are ext4 (4kB blocks with 4kB PAGE_SIZE),
>> and so is the filesystem I'm testing, ext4 on /dev/loop0 on tmpfs.
>> So, plenty of ext4 page cache and buffer_heads.
>>
>> Again and again, the deadlock is seen with buffer_migrate_folio_norefs(),
>> either in kcompactd0 or in khugepaged trying to compact, or in both:
>> it ends up calling __lock_buffer(), and that schedules away, waiting
>> forever to get BH_lock. I have not identified who is holding BH_lock,
>> but I imagine a jbd2 journalling thread, and presume that it wants one
>> of the folio locks which migrate_pages_batch() is already holding; or
>> maybe it's all more convoluted than that. Other tasks then back up
>> waiting on those folio locks held in the batch.
>>
>> Never a problem with buffer_migrate_folio(), always with the "more
>> careful" buffer_migrate_folio_norefs(). And the patch below fixes
>> it for me: I've had enough hours with it now, on enough occasions,
>> to be confident of that.
>>
>> Cc'ing Jan Kara, who knows buffer_migrate_folio_norefs() and jbd2
>> very well, and I hope can assure us that there is an understandable
>> deadlock here, from holding several random folio locks, then trying
>> to lock buffers. Cc'ing fsdevel, because there's a risk that mm
>> folk think something is safe, when it's not sufficient to cope with
>> the diversity of filesystems. I hope nothing more than the below is
>> needed (and I've had no other problems with the patchset: good job),
>> but cannot be sure.
>
> I suspect it can indeed be caused by the presence of the loop device as
> Huang Ying has suggested. What filesystems using buffer_heads do is a
> pattern like:
>
> bh = page_buffers(loop device page cache page);
> lock_buffer(bh);
> submit_bh(bh);
> - now on loop dev this ends up doing:
> lo_write_bvec()
> vfs_iter_write()
> ...
> folio_lock(backing file folio);
>
> So if migration code holds "backing file folio" lock and at the same time
> waits for 'bh' lock (while trying to migrate loop device page cache page), it
> is a deadlock.
>
> Proposed solution of never waiting for locks in batched mode looks like a
> sensible one to me...
Thank you very much for detail explanation!
Best Regards,
Huang, Ying
next prev parent reply other threads:[~2023-02-28 1:14 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-13 12:34 [PATCH -v5 0/9] migrate_pages(): batch TLB flushing Huang Ying
2023-02-13 12:34 ` [PATCH -v5 1/9] migrate_pages: organize stats with struct migrate_pages_stats Huang Ying
2023-02-13 12:34 ` [PATCH -v5 2/9] migrate_pages: separate hugetlb folios migration Huang Ying
2023-02-13 12:34 ` [PATCH -v5 3/9] migrate_pages: restrict number of pages to migrate in batch Huang Ying
2023-02-13 12:34 ` [PATCH -v5 4/9] migrate_pages: split unmap_and_move() to _unmap() and _move() Huang Ying
2023-02-13 12:34 ` [PATCH -v5 5/9] migrate_pages: batch _unmap and _move Huang Ying
2023-02-13 12:34 ` [PATCH -v5 6/9] migrate_pages: move migrate_folio_unmap() Huang Ying
2023-02-13 12:34 ` [PATCH -v5 7/9] migrate_pages: share more code between _unmap and _move Huang Ying
2023-02-13 12:34 ` [PATCH -v5 8/9] migrate_pages: batch flushing TLB Huang Ying
2023-02-13 12:34 ` [PATCH -v5 9/9] migrate_pages: move THP/hugetlb migration support check to simplify code Huang Ying
2023-02-17 21:47 ` [PATCH -v5 0/9] migrate_pages(): batch TLB flushing Hugh Dickins
2023-02-20 9:28 ` Huang, Ying
2023-02-21 2:48 ` Hugh Dickins
2023-02-21 3:34 ` Huang, Ying
2023-02-21 22:15 ` Hugh Dickins
2023-02-21 4:30 ` Matthew Wilcox
2023-02-21 4:38 ` Huang, Ying
2023-02-21 14:04 ` Huang, Ying
2023-02-21 22:25 ` Hugh Dickins
2023-02-22 1:02 ` Huang, Ying
2023-02-27 11:06 ` Jan Kara
2023-02-28 1:13 ` Huang, Ying [this message]
2023-02-28 5:59 ` Hugh Dickins
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