Re: [PATCH v2 0/5] Introduce DMA_HEAP_ALLOC_AND_READ_FILE heap flag

Huan Yang
2024-07-30T08:14:14Z

在 2024/7/30 16:03, Christian König 写道:
> Am 30.07.24 um 09:57 schrieb Huan Yang:
>> Background
>> ====
>> Some user may need load file into dma-buf, current way is:
>>    1. allocate a dma-buf, get dma-buf fd
>>    2. mmap dma-buf fd into user vaddr
>>    3. read(file_fd, vaddr, fsz)
>> Due to dma-buf user map can't support direct I/O[1], the file read
>> must be buffer I/O.
>>
>> This means that during the process of reading the file into dma-buf,
>> page cache needs to be generated, and the corresponding content needs to
>> be first copied to the page cache before being copied to the dma-buf.
>>
>> This way worked well when reading relatively small files before, as
>> the page cache can cache the file content, thus improving performance.
>>
>> However, there are new challenges currently, especially as AI models are
>> becoming larger and need to be shared between DMA devices and the CPU
>> via dma-buf.
>>
>> For example, our 7B model file size is around 3.4GB. Using the
>> previous would mean generating a total of 3.4GB of page cache
>> (even if it will be reclaimed), and also requiring the copying of 3.4GB
>> of content between page cache and dma-buf.
>>
>> Due to the limited resources of system memory, files in the gigabyte
>> range
>> cannot persist in memory indefinitely, so this portion of page cache may
>> not provide much assistance for subsequent reads. Additionally, the
>> existence of page cache will consume additional system resources due to
>> the extra copying required by the CPU.
>>
>> Therefore, I think it is necessary for dma-buf to support direct I/O.
>>
>> However, direct I/O file reads cannot be performed using the buffer
>> mmaped by the user space for the dma-buf.[1]
>>
>> Here are some discussions on implementing direct I/O using dma-buf:
>>
>> mmap[1]
>> ---
>> dma-buf never support user map vaddr use of direct I/O.
>>
>> udmabuf[2]
>> ---
>> Currently, udmabuf can use the memfd method to read files into
>> dma-buf in direct I/O mode.
>>
>> However, if the size is large, the current udmabuf needs to adjust the
>> corresponding size_limit(default 64MB).
>> But using udmabuf for files at the 3GB level is not a very good
>> approach.
>> It needs to make some adjustments internally to handle this.[3] Or else,
>> fail create.
>>
>> But, it is indeed a viable way to enable dma-buf to support direct I/O.
>> However, it is necessary to initiate the file read after the memory
>> allocation
>> is completed, and handle race conditions carefully.
>>
>> sendfile/splice[4]
>> ---
>> Another way to enable dma-buf to support direct I/O is by implementing
>> splice_write/write_iter in the dma-buf file operations (fops) to adapt
>> to the sendfile method.
>> However, the current sendfile/splice calls are based on pipe. When using
>> direct I/O to read a file, the content needs to be copied to the buffer
>> allocated by the pipe (default 64KB), and then the dma-buf fops'
>> splice_write needs to be called to write the content into the dma-buf.
>> This approach requires serially reading the content of file pipe size
>> into the pipe buffer and then waiting for the dma-buf to be written
>> before reading the next one.(The I/O performance is relatively weak
>> under direct I/O.)
>> Moreover, due to the existence of the pipe buffer, even when using
>> direct I/O and not needing to generate additional page cache,
>> there still needs to be a CPU copy.
>>
>> copy_file_range[5]
>> ---
>> Consider of copy_file_range, It only supports copying files within the
>> same file system. Similarly, it is not very practical.
>>
>>
>> So, currently, there is no particularly suitable solution on VFS to
>> allow dma-buf to support direct I/O for large file reads.
>>
>> This patchset provides an idea to complete file reads when requesting a
>> dma-buf.
>>
>> Introduce DMA_HEAP_ALLOC_AND_READ_FILE heap flag
>> ===
>> This patch provides a method to immediately read the file content after
>> the dma-buf is allocated, and only returns the dma-buf file descriptor
>> after the file is fully read.
>>
>> Since the dma-buf file descriptor is not returned, no other thread can
>> access it except for the current thread, so we don't need to worry about
>> race conditions.
>
> That is a completely false assumption.
Can you provide a detailed explanation as to why this assumption is
incorrect? thanks.
>
>>
>> Map the dma-buf to the vmalloc area and initiate file reads in kernel
>> space, supporting both buffer I/O and direct I/O.
>>
>> This patch adds the DMA_HEAP_ALLOC_AND_READ heap_flag for user.
>> When a user needs to allocate a dma-buf and read a file, they should
>> pass this heap flag. As the size of the file being read is fixed,
>> there is no
>> need to pass the 'len' parameter. Instead, The file_fd needs to be
>> passed to
>> indicate to the kernel the file that needs to be read.
>>
>> The file open flag determines the mode of file reading.
>> But, please note that if direct I/O(O_DIRECT) is needed to read the
>> file,
>> the file size must be page aligned. (with patch 2-5, no need)
>>
>> Therefore, for the user, len and file_fd are mutually exclusive,
>> and they are combined using a union.
>>
>> Once the user obtains the dma-buf fd, the dma-buf directly contains the
>> file content.
>
> And I'm repeating myself, but this is a complete NAK from my side to
> this approach.
>
> We pointed out multiple ways of how to implement this cleanly and not
> by hacking functionality into the kernel which absolutely doesn't
> belong there.
In this patchset, I have provided performance comparisons of each of
these methods.  Can you please provide more opinions?
>
> Regards,
> Christian.
>
>>
>> Patch 1 implement it.
>>
>> Patch 2-5 provides an approach for performance improvement.
>>
>> The DMA_HEAP_ALLOC_AND_READ_FILE heap flag patch enables us to
>> synchronously read files using direct I/O.
>>
>> This approach helps to save CPU copying and avoid a certain degree of
>> memory thrashing (page cache generation and reclamation)
>>
>> When dealing with large file sizes, the benefits of this approach become
>> particularly significant.
>>
>> However, there are currently some methods that can improve performance,
>> not just save system resources:
>>
>> Due to the large file size, for example, a AI 7B model of around
>> 3.4GB, the
>> time taken to allocate DMA-BUF memory will be relatively long. Waiting
>> for the allocation to complete before reading the file will add to the
>> overall time consumption. Therefore, the total time for DMA-BUF
>> allocation and file read can be calculated using the formula
>>     T(total) = T(alloc) + T(I/O)
>>
>> However, if we change our approach, we don't necessarily need to wait
>> for the DMA-BUF allocation to complete before initiating I/O. In fact,
>> during the allocation process, we already hold a portion of the page,
>> which means that waiting for subsequent page allocations to complete
>> before carrying out file reads is actually unfair to the pages that have
>> already been allocated.
>>
>> The allocation of pages is sequential, and the reading of the file is
>> also sequential, with the content and size corresponding to the file.
>> This means that the memory location for each page, which holds the
>> content of a specific position in the file, can be determined at the
>> time of allocation.
>>
>> However, to fully leverage I/O performance, it is best to wait and
>> gather a certain number of pages before initiating batch processing.
>>
>> The default gather size is 128MB. So, ever gathered can see as a file
>> read
>> work, it maps the gather page to the vmalloc area to obtain a continuous
>> virtual address, which is used as a buffer to store the contents of the
>> corresponding file. So, if using direct I/O to read a file, the file
>> content will be written directly to the corresponding dma-buf buffer
>> memory
>> without any additional copying.(compare to pipe buffer.)
>>
>> Consider other ways to read into dma-buf. If we assume reading after
>> mmap
>> dma-buf, we need to map the pages of the dma-buf to the user virtual
>> address space. Also, udmabuf memfd need do this operations too.
>> Even if we support sendfile, the file copy also need buffer, you must
>> setup it.
>> So, mapping pages to the vmalloc area does not incur any additional
>> performance overhead compared to other methods.[6]
>>
>> Certainly, the administrator can also modify the gather size through
>> patch5.
>>
>> The formula for the time taken for system_heap buffer allocation and
>> file reading through async_read is as follows:
>>
>>    T(total) = T(first gather page) + Max(T(remain alloc), T(I/O))
>>
>> Compared to the synchronous read:
>>    T(total) = T(alloc) + T(I/O)
>>
>> If the allocation time or I/O time is long, the time difference will be
>> covered by the maximum value between the allocation and I/O. The other
>> party will be concealed.
>>
>> Therefore, the larger the size of the file that needs to be read, the
>> greater the corresponding benefits will be.
>>
>> How to use
>> ===
>> Consider the current pathway for loading model files into DMA-BUF:
>>    1. open dma-heap, get heap fd
>>    2. open file, get file_fd(can't use O_DIRECT)
>>    3. use file len to allocate dma-buf, get dma-buf fd
>>    4. mmap dma-buf fd, get vaddr
>>    5. read(file_fd, vaddr, file_size) into dma-buf pages
>>    6. share, attach, whatever you want
>>
>> Use DMA_HEAP_ALLOC_AND_READ_FILE JUST a little change:
>>    1. open dma-heap, get heap fd
>>    2. open file, get file_fd(buffer/direct)
>>    3. allocate dma-buf with DMA_HEAP_ALLOC_AND_READ_FILE heap flag,
>> set file_fd
>>       instead of len. get dma-buf fd(contains file content)
>>    4. share, attach, whatever you want
>>
>> So, test it is easy.
>>
>> How to test
>> ===
>> The performance comparison will be conducted for the following
>> scenarios:
>>    1. normal
>>    2. udmabuf with [3] patch
>>    3. sendfile
>>    4. only patch 1
>>    5. patch1 - patch4.
>>
>> normal:
>>    1. open dma-heap, get heap fd
>>    2. open file, get file_fd(can't use O_DIRECT)
>>    3. use file len to allocate dma-buf, get dma-buf fd
>>    4. mmap dma-buf fd, get vaddr
>>    5. read(file_fd, vaddr, file_size) into dma-buf pages
>>    6. share, attach, whatever you want
>>
>> UDMA-BUF step:
>>    1. memfd_create
>>    2. open file(buffer/direct)
>>    3. udmabuf create
>>    4. mmap memfd
>>    5. read file into memfd vaddr
>>
>> Sendfile step(need suit splice_write/write_iter, just use to compare):
>>    1. open dma-heap, get heap fd
>>    2. open file, get file_fd(buffer/direct)
>>    3. use file len to allocate dma-buf, get dma-buf fd
>>    4. sendfile file_fd to dma-buf fd
>>    6. share, attach, whatever you want
>>
>> patch1/patch1-4:
>>    1. open dma-heap, get heap fd
>>    2. open file, get file_fd(buffer/direct)
>>    3. allocate dma-buf with DMA_HEAP_ALLOC_AND_READ_FILE heap flag,
>> set file_fd
>>       instead of len. get dma-buf fd(contains file content)
>>    4. share, attach, whatever you want
>>
>> You can create a file to test it. Compare the performance gap between
>> the two.
>> It is best to compare the differences in file size from KB to MB to GB.
>>
>> The following test data will compare the performance differences
>> between 512KB,
>> 8MB, 1GB, and 3GB under various scenarios.
>>
>> Performance Test
>> ===
>>    12G RAM phone
>>    UFS4.0(the maximum speed is 4GB/s. ),
>>    f2fs
>>    kernel 6.1 with patch[7] (or else, can't support kvec direct I/O
>> read.)
>>    no memory pressure.
>>    drop_cache is used for each test.
>>
>> The average of 5 test results:
>> | scheme-size         | 512KB(ns)  | 8MB(ns)    | 1GB(ns) |
>> 3GB(ns)       |
>> | ------------------- | ---------- | ---------- | ------------- |
>> ------------- |
>> | normal              | 2,790,861  | 14,535,784 | 1,520,790,492 |
>> 3,332,438,754 |
>> | udmabuf buffer I/O  | 1,704,046  | 11,313,476 | 821,348,000 |
>> 2,108,419,923 |
>> | sendfile buffer I/O | 3,261,261  | 12,112,292 | 1,565,939,938 |
>> 3,062,052,984 |
>> | patch1-4 buffer I/O | 2,064,538  | 10,771,474 | 986,338,800 |
>> 2,187,570,861 |
>> | sendfile direct I/O | 12,844,231 | 37,883,938 | 5,110,299,184 |
>> 9,777,661,077 |
>> | patch1 direct I/O   | 813,215    | 6,962,092  | 2,364,211,877 |
>> 5,648,897,554 |
>> | udmabuf direct I/O  | 1,289,554  | 8,968,138  | 921,480,784 |
>> 2,158,305,738 |
>> | patch1-4 direct I/O | 1,957,661  | 6,581,999  | 520,003,538 |
>> 1,400,006,107 |

With this test, sendfile can't give a good help base on pipe buffer.

udmabuf is good, but I think our oem driver can't suit it. (And, AOSP do
not open this feature)

Anyway, I am sending this patchset in the hope of further discussion.

Thanks.

>>
>> So, based on the test results:
>>
>> When the file is large, the patchset has the highest performance.
>> Compared to normal, patchset is a 50% improvement;
>> Compared to normal, patch1 only showed a degradation of 41%.
>> patch1 typical performance breakdown is as follows:
>>    1. alloc cost 188,802,693 ns
>>    2. vmap cost 42,491,385 ns
>>    3. file read cost 4,180,876,702 ns
>> Therefore, directly performing a single direct I/O read on a large file
>> may not be the most optimal way for performance.
>>
>> The performance of direct I/O implemented by the sendfile method is
>> the worst.
>>
>> When file size is small, The difference in performance is not
>> significant. This is consistent with expectations.
>>
>>
>>
>> Suggested use cases
>> ===
>>    1. When there is a need to read large files and system resources
>> are scarce,
>>       especially when the size of memory is limited.(GB level) In this
>>       scenario, using direct I/O for file reading can even bring
>> performance
>>       improvements.(may need patch2-3)
>>    2. For embedded devices with limited RAM, using direct I/O can
>> save system
>>       resources and avoid unnecessary data copying. Therefore, even
>> if the
>>       performance is lower when read small file, it can still be used
>>       effectively.
>>    3. If there is sufficient memory, pinning the page cache of the
>> model files
>>       in memory and placing file in the EROFS file system for
>> read-only access
>>       maybe better.(EROFS do not support direct I/O)
>>
>>
>> Changlog
>> ===
>>   v1 [8]
>>   v1->v2:
>>     Uses the heap flag method for alloc and read instead of adding a new
>>     DMA-buf ioctl command. [9]
>>     Split the patchset to facilitate review and test.
>>       patch 1 implement alloc and read, offer heap flag into it.
>>       patch 2-4 offer async read
>>       patch 5 can change gather limit.
>>
>> Reference
>> ===
>> [1]
>> https://lore.kernel.org/all/0393cf47-3fa2-4e32-8b3d-d5d5bdece298@amd.com/
>> [2]
>> https://lore.kernel.org/all/ZpTnzkdolpEwFbtu@phenom.ffwll.local/
>> [3]
>> https://lore.kernel.org/all/20240725021349.580574-1-link@vivo.com/
>> [4]
>> https://lore.kernel.org/all/Zpf5R7fRZZmEwVuR@infradead.org/
>> [5]
>> https://lore.kernel.org/all/ZpiHKY2pGiBuEq4z@infradead.org/
>> [6]
>> https://lore.kernel.org/all/9b70db2e-e562-4771-be6b-1fa8df19e356@amd.com/
>> [7]
>> https://patchew.org/linux/20230209102954.528942-1-dhowells@redhat.com/20230209102954.528942-7-dhowells@redhat.com/
>> [8]
>> https://lore.kernel.org/all/20240711074221.459589-1-link@vivo.com/
>> [9]
>> https://lore.kernel.org/all/5ccbe705-883c-4651-9e66-6b452c414c74@amd.com/
>>
>> Huan Yang (5):
>>    dma-buf: heaps: Introduce DMA_HEAP_ALLOC_AND_READ_FILE heap flag
>>    dma-buf: heaps: Introduce async alloc read ops
>>    dma-buf: heaps: support alloc async read file
>>    dma-buf: heaps: system_heap alloc support async read
>>    dma-buf: heaps: configurable async read gather limit
>>
>>   drivers/dma-buf/dma-heap.c          | 552 +++++++++++++++++++++++++++-
>>   drivers/dma-buf/heaps/system_heap.c |  70 +++-
>>   include/linux/dma-heap.h            |  53 ++-
>>   include/uapi/linux/dma-heap.h       |  11 +-
>>   4 files changed, 673 insertions(+), 13 deletions(-)
>>
>>
>> base-commit: 931a3b3bccc96e7708c82b30b2b5fa82dfd04890
>