[PATCH 2/2] dmabuf/heaps: implement DMA_BUF_IOCTL_RW_FILE for system_heap

[T.J. Mercier]

On Fri, May 16, 2025 at 1:36 AM Christian König
<christian.koenig at amd.com> wrote:
>
> On 5/16/25 09:40, wangtao wrote:
> >
> >
> >> -----Original Message-----
> >> From: Christian König <christian.koenig at amd.com>
> >> Sent: Thursday, May 15, 2025 10:26 PM
> >> To: wangtao <tao.wangtao at honor.com>; sumit.semwal at linaro.org;
> >> benjamin.gaignard at collabora.com; Brian.Starkey at arm.com;
> >> jstultz at google.com; tjmercier at google.com
> >> Cc: linux-media at vger.kernel.org; dri-devel at lists.freedesktop.org; linaro-
> >> mm-sig at lists.linaro.org; linux-kernel at vger.kernel.org;
> >> wangbintian(BintianWang) <bintian.wang at honor.com>; yipengxiang
> >> <yipengxiang at honor.com>; liulu 00013167 <liulu.liu at honor.com>; hanfeng
> >> 00012985 <feng.han at honor.com>
> >> Subject: Re: [PATCH 2/2] dmabuf/heaps: implement
> >> DMA_BUF_IOCTL_RW_FILE for system_heap
> >>
> >> On 5/15/25 16:03, wangtao wrote:
> >>> [wangtao] My Test Configuration (CPU 1GHz, 5-test average):
> >>> Allocation: 32x32MB buffer creation
> >>> - dmabuf 53ms vs. udmabuf 694ms (10X slower)
> >>> - Note: shmem shows excessive allocation time
> >>
> >> Yeah, that is something already noted by others as well. But that is
> >> orthogonal.
> >>
> >>>
> >>> Read 1024MB File:
> >>> - dmabuf direct 326ms vs. udmabuf direct 461ms (40% slower)
> >>> - Note: pin_user_pages_fast consumes majority CPU cycles
> >>>
> >>> Key function call timing: See details below.
> >>
> >> Those aren't valid, you are comparing different functionalities here.
> >>
> >> Please try using udmabuf with sendfile() as confirmed to be working by T.J.
> > [wangtao] Using buffer IO with dmabuf file read/write requires one memory copy.
> > Direct IO removes this copy to enable zero-copy. The sendfile system call
> > reduces memory copies from two (read/write) to one. However, with udmabuf,
> > sendfile still keeps at least one copy, failing zero-copy.
>
>
> Then please work on fixing this.
>
> Regards,
> Christian.
>
>
> >
> > If udmabuf sendfile uses buffer IO (file page cache), read latency matches
> > dmabuf buffer read, but allocation time is much longer.
> > With Direct IO, the default 16-page pipe size makes it slower than buffer IO.
> >
> > Test data shows:
> > udmabuf direct read is much faster than udmabuf sendfile.
> > dmabuf direct read outperforms udmabuf direct read by a large margin.
> >
> > Issue: After udmabuf is mapped via map_dma_buf, apps using memfd or
> > udmabuf for Direct IO might cause errors, but there are no safeguards to
> > prevent this.
> >
> > Allocate 32x32MB buffer and read 1024 MB file Test:
> > Metric                 | alloc (ms) | read (ms) | total (ms)
> > -----------------------|------------|-----------|-----------
> > udmabuf buffer read    | 539        | 2017      | 2555
> > udmabuf direct read    | 522        | 658       | 1179

I can't reproduce the part where udmabuf direct reads are faster than
buffered reads. That's the opposite of what I'd expect. Something
seems wrong with those buffered reads.

> > udmabuf buffer sendfile| 505        | 1040      | 1546
> > udmabuf direct sendfile| 510        | 2269      | 2780

I can reproduce the 3.5x slower udambuf direct sendfile compared to
udmabuf direct read. It's a pretty disappointing result, so it seems
like something could be improved there.

1G from ext4 on 6.12.17 | read/sendfile (ms)
------------------------|-------------------
udmabuf buffer read     | 351
udmabuf direct read     | 540
udmabuf buffer sendfile | 255
udmabuf direct sendfile | 1990


> > dmabuf buffer read     | 51         | 1068      | 1118
> > dmabuf direct read     | 52         | 297       | 349
> >
> > udmabuf sendfile test steps:
> > 1. Open data file(1024MB), get back_fd
> > 2. Create memfd(32MB) # Loop steps 2-6
> > 3. Allocate udmabuf with memfd
> > 4. Call sendfile(memfd, back_fd)
> > 5. Close memfd after sendfile
> > 6. Close udmabuf
> > 7. Close back_fd
> >
> >>
> >> Regards,
> >> Christian.
> >
>