Poor performace on mmap reading arm64 on audio device

[Takashi Iwai]

On Mon, 23 Nov 2020 14:44:52 +0100,
Michael Nazzareno Trimarchi wrote:
> 
> Hi
> 
> On Mon, Nov 23, 2020 at 2:23 PM Takashi Iwai <tiwai at suse.de> wrote:
> >
> > On Sat, 21 Nov 2020 10:40:04 +0100,
> > Michael Nazzareno Trimarchi wrote:
> > >
> > > Hi all
> > >
> > > I'm trying to figure out how to increase performance on audio reading
> > > using the mmap interface. Right now what I understand it's that
> > > allocation comes from core/memalloc.c ops that allocate the memory for
> > > dma under driver/dma.
> > > The reference platform I have is an imx8mm and the allocation in arm64 is:
> > >
> > > 0xffff800011ff5000-0xffff800012005000          64K PTE       RW NX SHD
> > > AF            UXN MEM/NORMAL-NC
> > >
> > > This is the reason that is allocated for dma interface.
> > >
> > > Now access linear on the multichannel interface the performance is bad
> > > but worse if I try to access a channel a time on read.
> > > So it looks like it is better to copy the block using memcpy on a
> > > cached area and then operate on a single channel sample. If it's
> > > correct what I'm saying the mmap_begin and mmap_commit
> > > basically they don't do anything on cache level so the page mapping
> > > and way is used is always the same. Can the interface be modified to
> > > allow cache the area during read and restore in the commit
> > > phase?
> >
> > The current API of the mmap for the sound ring-buffer is designed to
> > allow concurrent accesses at any time in the minimalistic kernel-user
> > context switching.  So the whole buffer is allocated as coherent and
> > mmapped in a shot.  It's pretty efficient for architectures like x86,
> > but has disadvantages on ARM, indeed.
> 
> Each platform e/o architecture can specialize the mmap and declare the
> area that is consistent in dma to me mapped
> as no cache one
> 
> vma->vm_page_prot = pgprot_cached(vma->vm_page_prot);
>                 return remap_pfn_range(vma, vma->vm_start,
>                                 vma->vm_end - vma->vm_start, vma->vm_page_prot);
> 
> I have done it for testing purposes. This give an idea
> 
> - read multi channel not sequentially took around 12% of the cpu with
> mmap interface
> - read multi channel use after a memcpy took around 6%
> - read on a cached area took around 3%. I'm trying to figure out how
> and when invalidate the area
> 
> I have two use cases:
> - write on the channels (no performance issue)
> - read on channels
> 
> Before reading I should only say that the cached area is not in sync
> with memory. I think that supporting write use cases
> makes little sense here.

It's a necessary use case, unfortunately.  The reason we ended up with
one device per direction for the PCM in many many years ago was that
some applications need to write the buffers for marking even for the
read.  So it can't be read-only, and it's supposed to be coherent on
both read and write -- as long as keeping the current API usage.

> > The mmap_begin and mmap_commit are the concepts in the alsa-lib side
> > for supporting the plugins better, and they doesn't represent kernel
> > ABI.  So, this extension would be needed at first, and the memory
> > allocation mechanism has to be changed as well.  Last but not least,
> 
> Are you sure about memory allocation, or just memory mapping?

I thought you'd need the proper memory allocation for the coherent
mmap?

> > the concurrency has to be reconsidered if this approach is taken.
> >
> 
> Yes I know that is a big problem anyway. I don't have a big idea how solve it

If you find a good solution, let us know.  It's a kind of historical
obstacle, but certainly it's solvable.


Takashi