CPU consumption is going as high as 95% on ARM Cortex A8

[Russell King - ARM Linux]

On Mon, Dec 21, 2009 at 02:51:13PM +0530, Hiremath, Vaibhav wrote:
> > On Mon, Dec 21, 2009 at 11:56:23AM +0530, Hiremath, Vaibhav wrote:
> > > >         vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> > > >
> > > > will result in the memory being mapped as 'Strongly Ordered',
> > > > resulting
> > > > in there being multiple mappings with differing types.  In later
> > > > kernels, we have pgprot_dmacoherent() and I'd suggest changing
> > the
> > > > above
> > > > macro for that.
> > > >
> > >
> > > I tried with your suggestion above but unfortunately it didn't
> > work for
> > > me. I am seeing the same behavior with the pgprot_dmacoherent(). I
> > > pulled your patch (which got applied cleanly on 2.6.32-rc5) -
> > 
> > What happens if you comment out the pgprot_dmacoherent() /
> > pgprot_noncached()
> > line completely?
>
> If I comment the line completely then I am seeing
> CPU consumption similar to when I was setting PAGE_READONLY/PAGE_SHARED
> flag, which is 25-32%.
> 
> > I suspect that will "solve" the problem - but you'll then no longer
> > have
> > DMA coherency with userspace, so its not really a solution.

So it _is_ down to purely the amount of time it takes to read from a
non-cacheable buffer.  I think you need to investigate the userspace
program and see whether it's doing anything silly - I don't think the
lack of performance is a kernel problem as such.

How large is this buffer?  What userspace program is reading from it?
Could the userspace program be unnecessarily re-reading from the
multiple times for the same frame?