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From: Yu Xu <xuyu@linux.alibaba.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Hillf Danton <hdanton@sina.com>, Hugh Dickins <hughd@google.com>,
	Josef Bacik <josef@toxicpanda.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Linux-MM <linux-mm@kvack.org>,
	mm-commits@vger.kernel.org, Will Deacon <will.deacon@arm.com>,
	Matthew Wilcox <willy@infradead.org>,
	yang.shi@linux.alibaba.com
Subject: Re: [patch 01/15] mm/memory.c: avoid access flag update TLB flush for retried page fault
Date: Tue, 28 Jul 2020 01:12:43 +0800
Message-ID: <39560818-463f-da3a-fc9e-3a4a0a082f61@linux.alibaba.com> (raw)
In-Reply-To: <20200727110512.GB25400@gaia>

On 7/27/20 7:05 PM, Catalin Marinas wrote:
> On Mon, Jul 27, 2020 at 03:31:16PM +0800, Yu Xu wrote:
>> On 7/25/20 4:22 AM, Linus Torvalds wrote:
>>> On Fri, Jul 24, 2020 at 12:27 PM Linus Torvalds
>>> <torvalds@linux-foundation.org> wrote:
>>>>
>>>> It *may* make sense to say "ok, don't bother flushing the TLB if this
>>>> is a retry, because we already did that originally". MAYBE.
> [...]
>>> We could say that we never need it at all for FAULT_FLAG_RETRY. That
>>> makes a lot of sense to me.
>>>
>>> So a patch that does something like the appended (intentionally
>>> whitespace-damaged) seems sensible.
>>
>> I tested your patch on our aarch64 box, with 128 online CPUs.
> [...]
>> There are two points to sum up.
>>
>> 1) the performance of page_fault3_process is restored, while the performance
>> of page_fault3_thread is about ~80% of the vanilla, except the case of 128
>> threads.
>>
>> 2) in the case of 128 threads, test worker threads seem to get stuck, making
>> no progress in the iterations of mmap-write-munmap until a period of time
>> later.  the test result is 0 because only first 16 samples are counted, and
>> they are all 0.  This situation is easy to re-produce with large number of
>> threads (not necessarily 128), and the stack of one stuck thread is shown
>> below.
>>
>> [<0>] __switch_to+0xdc/0x150
>> [<0>] wb_wait_for_completion+0x84/0xb0
>> [<0>] __writeback_inodes_sb_nr+0x9c/0xe8
>> [<0>] try_to_writeback_inodes_sb+0x6c/0x88
>> [<0>] ext4_nonda_switch+0x90/0x98 [ext4]
>> [<0>] ext4_page_mkwrite+0x248/0x4c0 [ext4]
>> [<0>] do_page_mkwrite+0x4c/0x100
>> [<0>] do_fault+0x2ac/0x3e0
>> [<0>] handle_pte_fault+0xb4/0x258
>> [<0>] __handle_mm_fault+0x1d8/0x3a8
>> [<0>] handle_mm_fault+0x104/0x1d0
>> [<0>] do_page_fault+0x16c/0x490
>> [<0>] do_translation_fault+0x60/0x68
>> [<0>] do_mem_abort+0x58/0x100
>> [<0>] el0_da+0x24/0x28
>> [<0>] 0xffffffffffffffff
>>
>> It seems quite normal, right? and I've run out of ideas.
> 
> If threads get stuck here, it could be a stale TLB entry that's not
> flushed with Linus' patch. Since that's a write fault, I think it hits
> the FAULT_FLAG_TRIED case.

There must be some changes in my test box, because I find that even the
vanilla kernel (89b15332af7c^) get result of 0 in 128t testcase.  And I
just directly used the history test data as the baseline.  I will dig
into this then.

And do we still need to concern the ~20% performance drop in thread mode?

> 
> Could you give my patch here a try as an alternative:
> 
> https://lore.kernel.org/linux-mm/20200725155841.GA14490@gaia/

I ran the same test on the same aarch64 box, with your patch, the result
is as follows.

test          vanilla kernel      patched kernel
parameter     (89b15332af7c^)     (Catalin's patch)
1p            829299              787676    (96.36 %)
1t            998007              789284    (78.36 %)
32p           18916718            17921100  (94.68 %)
32t           2020918             1644146   (67.64 %)
64p           18965168            18983580  (100.0 %)
64t           1415404             1093750   (48.03 %)
96p           18949438            18963921  (100.1 %)
96t           1622876             1262878   (63.72 %)
128p          18926813            1680146   (8.89  %)
128t          1643109             0 (0.00 % ) # ignore this temporarily

Thanks
Yu

> 
> It leaves the spurious flush in place but only local (though note that
> in a guest under KVM, all local TLBIs are upgraded to inner-shareable,
> so you'd not get the performance benefit).
> 


  parent reply index

Thread overview: 69+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-24  4:14 incoming Andrew Morton
2020-07-24  4:15 ` [patch 01/15] mm/memory.c: avoid access flag update TLB flush for retried page fault Andrew Morton
2020-07-24  4:38   ` Yang Shi
2020-07-24  4:56     ` Andrew Morton
2020-07-24 19:27   ` Linus Torvalds
2020-07-24 20:22     ` Linus Torvalds
2020-07-25  0:36       ` Yang Shi
2020-07-25  1:29         ` Linus Torvalds
2020-07-25 15:58           ` Catalin Marinas
2020-07-28  9:22             ` Will Deacon
2020-07-28  9:39               ` Catalin Marinas
2020-07-28 10:07                 ` Yu Xu
2020-07-28 11:46                   ` Catalin Marinas
2020-07-28 10:21                 ` Will Deacon
2020-07-28 18:28                 ` Linus Torvalds
2020-07-27 17:52           ` Yang Shi
2020-07-27 18:04             ` Linus Torvalds
2020-07-27 18:42               ` Catalin Marinas
2020-07-27 20:56                 ` Linus Torvalds
2020-07-27 22:34               ` Yang Shi
2020-07-27  7:31       ` Yu Xu
2020-07-27 11:05         ` Catalin Marinas
2020-07-27 17:01           ` Linus Torvalds
2020-07-28 11:19             ` Catalin Marinas
2020-07-27 17:12           ` Yu Xu [this message]
2020-07-27 18:04             ` Yang Shi
2020-07-27 18:37               ` Linus Torvalds
2020-07-27 22:43                 ` Yang Shi
2020-07-28  0:38                   ` Linus Torvalds
2020-07-28  0:13                 ` Yu Xu
2020-07-28 10:53                 ` Nicholas Piggin
2020-07-28 19:02                   ` Linus Torvalds
2020-07-28 22:53                     ` Nicholas Piggin
2020-07-29 13:58                       ` Michael Ellerman
2020-07-28  6:41             ` Yu Xu
2020-07-24  4:15 ` [patch 02/15] mm/mmap.c: close race between munmap() and expand_upwards()/downwards() Andrew Morton
2020-07-24  4:15 ` [patch 03/15] vfs/xattr: mm/shmem: kernfs: release simple xattr entry in a right way Andrew Morton
2020-07-24  4:15 ` [patch 04/15] mm: initialize return of vm_insert_pages Andrew Morton
2020-07-24  4:15 ` [patch 05/15] mm/memcontrol: fix OOPS inside mem_cgroup_get_nr_swap_pages() Andrew Morton
2020-07-24  4:15 ` [patch 06/15] mm/memcg: fix refcount error while moving and swapping Andrew Morton
2020-07-24 13:41   ` Alex Shi
2020-07-24  4:15 ` [patch 07/15] mm: memcg/slab: fix memory leak at non-root kmem_cache destroy Andrew Morton
2020-07-24  4:15 ` [patch 08/15] mm/hugetlb: avoid hardcoding while checking if cma is enabled Andrew Morton
2020-07-24  4:15 ` [patch 09/15] khugepaged: fix null-pointer dereference due to race Andrew Morton
2020-07-24  4:15 ` [patch 10/15] mailmap: add entry for Mike Rapoport Andrew Morton
2020-07-24  4:15 ` [patch 11/15] squashfs: fix length field overlap check in metadata reading Andrew Morton
2020-07-24  4:15 ` [patch 12/15] scripts/decode_stacktrace: strip basepath from all paths Andrew Morton
2020-07-24  4:15 ` [patch 13/15] io-mapping: indicate mapping failure Andrew Morton
2020-07-24  4:15 ` [patch 14/15] MAINTAINERS: add KCOV section Andrew Morton
2020-07-24  4:15 ` [patch 15/15] scripts/gdb: fix lx-symbols 'gdb.error' while loading modules Andrew Morton
2020-07-28  1:19 ` mmotm 2020-07-27-18-18 uploaded Andrew Morton
2020-07-28  2:14   ` Stephen Rothwell
2020-07-28  3:22   ` mmotm 2020-07-27-18-18 uploaded (drivers/scsi/ufs/: SCSI_UFS_EXYNOS) Randy Dunlap
2020-07-28  8:23     ` Alim Akhtar
2020-07-28 12:33   ` mmotm 2020-07-27-18-18 uploaded (mm/page_alloc.c) Randy Dunlap
2020-07-28 21:55     ` Andrew Morton
2020-07-28 22:20       ` Stephen Rothwell
2020-07-28 22:31         ` Andrew Morton
2020-07-29 14:18           ` Michael S. Tsirkin
2020-07-29 14:38             ` David Hildenbrand
2020-07-29 16:14               ` David Hildenbrand
2020-07-29 17:29                 ` Randy Dunlap
2020-07-28 22:39       ` Randy Dunlap
2020-07-29  1:43         ` Nathan Chancellor
2020-07-29  1:44         ` Andrew Morton
2020-07-29  2:04           ` Randy Dunlap
2020-07-29 14:09           ` make oldconfig (Re: mmotm 2020-07-27-18-18 uploaded (mm/page_alloc.c)) Alexey Dobriyan
2020-07-31 23:46 ` mmotm 2020-07-31-16-45 uploaded Andrew Morton
2020-08-01  5:24   ` mmotm 2020-07-31-16-45 uploaded (drivers/staging/vc04_services/) Randy Dunlap

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