dma_sync_sg_for_device/cpu look very inefficient

dma_sync_sg_for_device/cpu look very inefficient

[Paul Cercueil]

Hi,

I am manipulating DMABUFs created with the udmabuf driver, that are 64
MiB each, on a ZedBoard (which should have ~512 KiB of L2 cache IIRC).

When trying to access them with the CPU, dma_sync_sg_for_cpu() is
called. What ends up happening, is a cache sync for each one of the (up
to) 16 thousand pages that back up the DMABUF, which obviously takes a
huge amount of time, tanking performance.

My guess (probably very naïve) is that if the total length of the SG is
equal or bigger than the size of the wider non-coherent cache, it would
be much better to just flush the whole data cache.

Thoughts?

Cheers,
-Paul

Re: dma_sync_sg_for_device/cpu look very inefficient

[Christoph Hellwig]

On Mon, Dec 18, 2023 at 01:32:59PM +0100, Paul Cercueil wrote:
> I am manipulating DMABUFs created with the udmabuf driver, that are 64
> MiB each, on a ZedBoard (which should have ~512 KiB of L2 cache IIRC).
> 
> When trying to access them with the CPU, dma_sync_sg_for_cpu() is
> called. What ends up happening, is a cache sync for each one of the (up
> to) 16 thousand pages that back up the DMABUF, which obviously takes a
> huge amount of time, tanking performance.
> 
> My guess (probably very naïve) is that if the total length of the SG is
> equal or bigger than the size of the wider non-coherent cache, it would
> be much better to just flush the whole data cache.

IFF we regularly ѕync huge amount of data a complete flush is probably
going to be more efficient.  But why are we doing that to start with?
Ownership needs to transfer when the data is accessed, and when you
regularly access 16.000 pages you're probably got other performance
problems to start with.

Re: dma_sync_sg_for_device/cpu look very inefficient

[Lucas Stach]

Am Montag, dem 18.12.2023 um 15:30 +0100 schrieb Christoph Hellwig:
> On Mon, Dec 18, 2023 at 01:32:59PM +0100, Paul Cercueil wrote:
> > I am manipulating DMABUFs created with the udmabuf driver, that are 64
> > MiB each, on a ZedBoard (which should have ~512 KiB of L2 cache IIRC).
> > 
> > When trying to access them with the CPU, dma_sync_sg_for_cpu() is
> > called. What ends up happening, is a cache sync for each one of the (up
> > to) 16 thousand pages that back up the DMABUF, which obviously takes a
> > huge amount of time, tanking performance.
> > 
> > My guess (probably very naïve) is that if the total length of the SG is
> > equal or bigger than the size of the wider non-coherent cache, it would
> > be much better to just flush the whole data cache.
> 
> IFF we regularly ѕync huge amount of data a complete flush is probably
> going to be more efficient.  But why are we doing that to start with?
> Ownership needs to transfer when the data is accessed, and when you
> regularly access 16.000 pages you're probably got other performance
> problems to start with.
> 
In addition to that flushing the cache only works for data moving to
the device. When you have to deal with data written by the device you
would need to invalidate the cache, which is obviously not something
you can do to the whole cache without data corruption.

Regards,
Lucas

Re: dma_sync_sg_for_device/cpu look very inefficient

[Paul Cercueil]

Hi Lucas,

Le lundi 18 décembre 2023 à 15:40 +0100, Lucas Stach a écrit :
> Am Montag, dem 18.12.2023 um 15:30 +0100 schrieb Christoph Hellwig:
> > On Mon, Dec 18, 2023 at 01:32:59PM +0100, Paul Cercueil wrote:
> > > I am manipulating DMABUFs created with the udmabuf driver, that
> > > are 64
> > > MiB each, on a ZedBoard (which should have ~512 KiB of L2 cache
> > > IIRC).
> > > 
> > > When trying to access them with the CPU, dma_sync_sg_for_cpu() is
> > > called. What ends up happening, is a cache sync for each one of
> > > the (up
> > > to) 16 thousand pages that back up the DMABUF, which obviously
> > > takes a
> > > huge amount of time, tanking performance.
> > > 
> > > My guess (probably very naïve) is that if the total length of the
> > > SG is
> > > equal or bigger than the size of the wider non-coherent cache, it
> > > would
> > > be much better to just flush the whole data cache.
> > 
> > IFF we regularly ѕync huge amount of data a complete flush is
> > probably
> > going to be more efficient.  But why are we doing that to start
> > with?
> > Ownership needs to transfer when the data is accessed, and when you
> > regularly access 16.000 pages you're probably got other performance
> > problems to start with.
> > 
> In addition to that flushing the cache only works for data moving to
> the device. When you have to deal with data written by the device you
> would need to invalidate the cache, which is obviously not something
> you can do to the whole cache without data corruption.

Ah, that's a very good point.

Cheers,
-Paul

Re: dma_sync_sg_for_device/cpu look very inefficient

[Paul Cercueil]

Hi Christoph,

Le lundi 18 décembre 2023 à 15:30 +0100, Christoph Hellwig a écrit :
> On Mon, Dec 18, 2023 at 01:32:59PM +0100, Paul Cercueil wrote:
> > I am manipulating DMABUFs created with the udmabuf driver, that are
> > 64
> > MiB each, on a ZedBoard (which should have ~512 KiB of L2 cache
> > IIRC).
> > 
> > When trying to access them with the CPU, dma_sync_sg_for_cpu() is
> > called. What ends up happening, is a cache sync for each one of the
> > (up
> > to) 16 thousand pages that back up the DMABUF, which obviously
> > takes a
> > huge amount of time, tanking performance.
> > 
> > My guess (probably very naïve) is that if the total length of the
> > SG is
> > equal or bigger than the size of the wider non-coherent cache, it
> > would
> > be much better to just flush the whole data cache.
> 
> IFF we regularly ѕync huge amount of data a complete flush is
> probably
> going to be more efficient.  But why are we doing that to start with?
> Ownership needs to transfer when the data is accessed, and when you
> regularly access 16.000 pages you're probably got other performance
> problems to start with.

I mmap the DMABUFs from user-space, so that I can pass huge amounts of
data back and forth between user-space and kernel/DMA space without
going through copy_to_user().

This works fine and is faster than copy_to_user() with coherent memory,
but it's much slower with non-coherent memory because of the time spent
in dma_sync_sg_for_cpu().

(Note that we also have regular use cases where we share the DMABUFs
between device drivers to transfer data in a zero-copy fashion - we're
not just abusing them as a kernel/userspace interface.)

You are right that the problem is more that I have an insane number of
pages :) Thanks for pointing that out. I will try to use huge pages
instead.

Cheers,
-Paul

Re: dma_sync_sg_for_device/cpu look very inefficient

[Robin Murphy]

On 2023-12-18 12:32 pm, Paul Cercueil wrote:
> Hi,
> 
> I am manipulating DMABUFs created with the udmabuf driver, that are 64
> MiB each, on a ZedBoard (which should have ~512 KiB of L2 cache IIRC).
> 
> When trying to access them with the CPU, dma_sync_sg_for_cpu() is
> called. What ends up happening, is a cache sync for each one of the (up
> to) 16 thousand pages that back up the DMABUF, which obviously takes a
> huge amount of time, tanking performance.
> 
> My guess (probably very naïve) is that if the total length of the SG is
> equal or bigger than the size of the wider non-coherent cache, it would
> be much better to just flush the whole data cache.

In addition to the points already raised, not all architectures have a 
notion of "the whole data cache", so while this is something that 
individual arch implementations could potentially consider, it's not 
necessarily possible (or at least reasonable) to do at the generic DMA 
API level, nor in common implementations like dma-direct or iommu-dma.

Thanks,
Robin.