Date: Mon, 26 Feb 2024 16:46:00 +0800
From: "zhangpeng (AS)" <zhangpeng362@...wei.com>
To: David Hildenbrand <david@...hat.com>, "Huang, Ying" <ying.huang@...el.com>
CC: <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	<akpm@...ux-foundation.org>, <willy@...radead.org>, <fengwei.yin@...el.com>,
	<aneesh.kumar@...ux.ibm.com>, <shy828301@...il.com>, <hughd@...gle.com>,
	<wangkefeng.wang@...wei.com>, Nanyong Sun <sunnanyong@...wei.com>
Subject: Re: [PATCH v2] filemap: avoid unnecessary major faults in
 filemap_fault()

On 2024/2/26 16:20, David Hildenbrand wrote:

> On 26.02.24 08:52, Huang, Ying wrote:
>> "zhangpeng (AS)" <zhangpeng362@...wei.com> writes:
>>
>>> On 2024/2/26 14:04, Huang, Ying wrote:
>>>
>>>> "zhangpeng (AS)" <zhangpeng362@...wei.com> writes:
>>>>
>>>>> On 2024/2/7 10:21, Huang, Ying wrote:
>>>>>
>>>>>> Peng Zhang <zhangpeng362@...wei.com> writes:
>>>>>>> From: ZhangPeng <zhangpeng362@...wei.com>
>>>>>>>
>>>>>>> The major fault occurred when using mlockall(MCL_CURRENT | 
>>>>>>> MCL_FUTURE)
>>>>>>> in application, which leading to an unexpected performance 
>>>>>>> issue[1].
>>>>>>>
>>>>>>> This caused by temporarily cleared PTE during a 
>>>>>>> read+clear/modify/write
>>>>>>> update of the PTE, eg, do_numa_page()/change_pte_range().
>>>>>>>
>>>>>>> For the data segment of the user-mode program, the global 
>>>>>>> variable area
>>>>>>> is a private mapping. After the pagecache is loaded, the private 
>>>>>>> anonymous
>>>>>>> page is generated after the COW is triggered. Mlockall can lock 
>>>>>>> COW pages
>>>>>>> (anonymous pages), but the original file pages cannot be locked 
>>>>>>> and may
>>>>>>> be reclaimed. If the global variable (private anon page) is 
>>>>>>> accessed when
>>>>>>> vmf->pte is zeroed in numa fault, a file page fault will be 
>>>>>>> triggered.
>>>>>>>
>>>>>>> At this time, the original private file page may have been 
>>>>>>> reclaimed.
>>>>>>> If the page cache is not available at this time, a major fault 
>>>>>>> will be
>>>>>>> triggered and the file will be read, causing additional overhead.
>>>>>>>
>>>>>>> Fix this by rechecking the PTE without acquiring PTL in 
>>>>>>> filemap_fault()
>>>>>>> before triggering a major fault.
>>>>>>>
>>>>>>> Testing file anonymous page read and write page fault 
>>>>>>> performance in ext4
>>>>>>> and ramdisk using will-it-scale[2] on a x86 physical machine. 
>>>>>>> The data
>>>>>>> is the average change compared with the mainline after the patch is
>>>>>>> applied. The test results are within the range of fluctuation, 
>>>>>>> and there
>>>>>>> is no obvious difference. The test results are as follows:
>>>>>> You still claim that there's no difference in the test results.  
>>>>>> If so,
>>>>>> why do you implement the patch?  IMHO, you need to prove your 
>>>>>> patch can
>>>>>> improve the performance in some cases.
>>>>> I'm sorry that maybe I didn't express myself clearly.
>>>>>
>>>>> The purpose of this patch is to fix the issue that major fault may 
>>>>> still be triggered
>>>>> with mlockall(), thereby improving a little performance. This 
>>>>> patch is more of a bugfix
>>>>> than a performance improvement patch.
>>>>>
>>>>> This issue affects our traffic analysis service. The inbound 
>>>>> traffic is heavy. If a major
>>>>> fault occurs, the I/O schedule is triggered and the original I/O 
>>>>> is suspended. Generally,
>>>>> the I/O schedule is 0.7 ms. If other applications are operating 
>>>>> disks, the system needs
>>>>> to wait for more than 10 ms. However, the inbound traffic is heavy 
>>>>> and the NIC buffer is
>>>>> small. As a result, packet loss occurs. The traffic analysis 
>>>>> service can't tolerate packet
>>>>> loss.
>>>>>
>>>>> To prevent packet loss, we use the mlockall() function to prevent 
>>>>> I/O. It is unreasonable
>>>>> that major faults will still be triggered after mlockall() is used.
>>>>>
>>>>> In our service test environment, the baseline is 7 major faults/12 
>>>>> hours. After applied the
>>>>> unlock patch, the probability of triggering the major fault is 1 
>>>>> major faults/12 hours. After
>>>>> applied the lock patch, no major fault will be triggered. So only 
>>>>> the locked patch can actually
>>>>> solve our problem.
>>>> This is the data I asked for.
>>>>
>>>> But, you said that this is a feature bug fix instead of performance
>>>> improvement.  So, I checked the mlock(2), and found the following 
>>>> words,
>>>>
>>>> "
>>>>          mlockall() locks all pages mapped into the address space 
>>>> of the calling
>>>>          process.  This includes the pages of the code, data, and 
>>>> stack segment,
>>>>          as well as shared libraries, user space kernel data, 
>>>> shared memory, and
>>>>          memory-mapped files.  All mapped pages are guaranteed to 
>>>> be resident in
>>>>          RAM when the call returns successfully; the  pages are  
>>>> guaranteed  to
>>>>          stay in RAM until later unlocked.
>>>> "
>>>>
>>>> In theory, the locked page are in RAM.  So, IIUC, we don't violate the
>>>> ABI.  But, in effect, we does do that.
>>>
>>> "mlockall() locks all pages mapped into the address space of the 
>>> calling process."
>>> For a private mapping, mlockall() can lock COW pages (anonymous 
>>> pages), but the
>>> original file pages can't be locked. Maybe, we violate the ABI here.
>>
>> If so, please make it explicit and loudly.
>>
>>> This is also
>>> the cause of this issue. The patch fix the impact of this issue: 
>>> prevent major
>>> faults, reduce IO, and fix the service packet loss issue.
>>>
>>> Preventing major faults, and thus reducing IO, could be an important 
>>> reason to use
>>> mlockall(). Could we fix this with the locked patch? Or is there 
>>> another way?
>>
>> Unfortunately, locked patch cause performance regressions for more
>> common cases.  Is it possible for us to change ptep_modify_prot_start()
>> to use some magic PTE value instead of 0?  That may be possible.  But,
>> that isn't enough, you need to change all ptep_get_and_clear() users.
>
> Trigger (false) major faults for mlocked memory is suboptimal.
>
> Having such pages temporarily not mapped (e.g., page migration) is 
> acceptable (pages are in RAM but are getting moved). We handle that 
> using nonswap migration entries.
>
> Let me understand the issue first:
>
> 1) MAP_PRIVATE file mapping that is mlocked.
>
> 2) We caused COW, so we now have an anonymous page mapped. That anon
>    page is mlocked.
>
> 3) Change of protection (under PT lock) will temporarily clear the PTE
>
> 4) Page fault will trigger and find the PTE still cleared (without PT
>    lock)
>
> 5) We don't realize that there is a page mapped and, therefore, trigger
>    a major fault.
>
> Using the PT lock would fix it properly. Doing it as in this patch can 
> only be considered an optimization, not a proper fix.
>
> Using a magic PTE to work around "just use the PT lock like everyone 
> else" feels a bit odd. The patch states "We don't hold PTL here as 
> acquiring PTL hurts performance" -- do we have any numbers on that?
>
Testing file anonymous page read and write page fault performance in ext4
, tmpfs and ramdisk using will-it-scale[2] on a x86 physical machine. The
data is the average change compared with the mainline after the patch is
applied.

with the locked patch:
                           processes processes_idle threads threads_idle
ext4    private file write: -0.51%    0.08%          -0.03%  -0.04%
ext4    shared  file write:  0.135%  -0.531%          2.883% -0.772%
ramdisk private file write: -0.48%    0.23%          -1.08%   0.27%
ramdisk private file  read:  0.07%   -6.90%          -5.85%  -0.70%
tmpfs   private file write: -0.344%  -0.110%          0.200%  0.145%
tmpfs   shared  file write:  0.958%   0.101%          2.781% -0.337%
tmpfs    private file read: -0.16%    0.00%          -0.12%   0.41%

with the no locked patch:
                           processes processes_idle threads threads_idle
ext4    private file write: -1.14%  -0.08%         -1.87%   0.13%
ext4    private file  read:  0.03%  -0.65%         -0.51%  -0.08%
ramdisk private file write: -1.21%  -0.21%         -1.12%   0.11%
ramdisk private file  read:  0.00%  -0.68%         -0.33%  -0.02%

I could also run other tests if needed.

> We could special-case that for MLOCK'ed VMAs with MCL_FUTURE, meaning, 
> take the PTL to double-check only in such VMAs.
>
Agreed. I think this solution is great. Thanks for your suggestion!

-- 
Best Regards,
Peng

Powered by blists - more mailing lists