Re: [patch 01/15] mm/memory.c: avoid access flag update TLB flush for retried page fault

[Yu Xu <xuyu@linux.alibaba.com>]

On 7/28/20 1:12 AM, Yu Xu wrote:
> 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.

Hi all, I reset the test box, and re-run the whole test, the result this
time makes more sense.

Test  89b15332a^  Linus    Catalin    Yang
1p      100       90.10    79.20      86.19   %
1t      100       89.56    88.74      92.21   %
32p     100       98.22    97.36      98.91   %
32t     100       75.45    76.06      75.75   %
64p     100       99.97    100.01     99.97   %
64t     100       70.44    74.53      61.75   %
96p     100       99.95    99.91      100.00  %
96t     100       67.95    72.56      63.88   %
128p    100       99.92    99.93      100.12  %
128t    100       73.23    73.85      73.16   %

Sorry for previously confusing test data. Performance drop in thread
mode is now the remaining issue.

Thanks
Yu

> 
> 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).
>>