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

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

On 7/27/20 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).
>