Re: [PATCH v6 00/17] mm, dma, arm64: Reduce ARCH_KMALLOC_MINALIGN to 8

[Isaac Manjarres <isaacmanjarres@google.com>]

On Thu, Jun 08, 2023 at 10:05:58AM +0200, Ard Biesheuvel wrote:
> On Thu, 8 Jun 2023 at 07:45, Isaac Manjarres <isaacmanjarres@google.com> wrote:
> >
> > On Wed, May 31, 2023 at 8:48 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
> > > Here's version 6 of the series reducing the kmalloc() minimum alignment
> > > on arm64 to 8 (from 128). There are patches already to do the same for
> > > riscv (pretty straight-forward after this series).
> > Thanks, Catalin for getting these patches out. Please add my "Tested-by:" tag
> > for the series:
> >
> > Tested-by: Isaac J. Manjarres <isaacmanjarres@google.com>
> >
> > With the first 11 patches, I observed a reduction of 18.4 MB
> > in the slab memory footprint on my Pixel 6 device. After applying the
> > rest of the patches in the series, I observed a total reduction of
> > 26.5 MB in the
> > slab memory footprint on my device. These are great results!
> >
> 
> It would also be good to get an insight into how much bouncing is
> going on in this case, given that (AFAIK) Pixel 6 uses non-cache
> coherent DMA.

I enabled the "swiotlb_bounced" trace event from the kernel command line
to see if anything was being bounced. It turns out that for Pixel 6
there are non-coherent DMA transfers occurring, but none of the
transfers that are in either the DMA_FROM_DEVICE or
DMA_BIDIRECTIONAL directions are small enough to require bouncing.

--Isaac

P.S. I noticed that the trace_swiotlb_bounced() tracepoint may not be
invoked even though bouncing occurs. For example, in the dma-iommu path,
swiotlb_tbl_map_single() is called when bouncing, instead of
swiotlb_map(), which is what ends up calling trace_swiotlb_bounced().

Would it make sense to move the call to trace_swiotlb_bounced() to
swiotlb_tbl_map_single() since that function is always invoked?