Re: [PATCH] mm,page_alloc,cma: configurable CMA utilization

[Roman Gushchin <roman.gushchin@linux.dev>]

On Tue, Jan 31, 2023 at 08:06:28PM -0800, Chris Goldsworthy wrote:
> On Tue, Jan 31, 2023 at 03:59:36PM -0800, Roman Gushchin wrote:
> > On Tue, Jan 31, 2023 at 12:10:01PM -0800, Sukadev Bhattiprolu wrote:
> > > On Tue, Jan 31, 2023 at 10:10:40AM -0800, Roman Gushchin wrote:
> > > > Hi Sukadev!
> > > > 
> > > > Can you, please, share a bit more details about your setup? E.g. what is
> > > > the zone size, the cma area size and the value you want to set your sysctl to?
> > > 
> > > Hi Roman,
> > > 
> > > I currently have a device with 8GB Zone normal and 600MB of CMA. We have a
> > > slightly different implementation and use up all the available CMA region.
> > > i.e. going forward, we intend to set the ratio to 100 or even higher.
> 
> 
> Hi Roman,
> 
> > It means you want allocations be always served from a cma region first?
> 
> Exactly.
> 
> > What's the point of it?
> 
> We're operating in a resource constrained environment, and we want to maximize
> the amount of memory free / headroom for GFP_KERNEL allocations on our SoCs,
> which are especially important for DMA allocations that use an IOMMU. We need a
> large amount of CMA on our SoCs for various reasons (e.g. for devices not
> upstream of an IOMMU), but whilst that CMA memory is not in use, we want to
> route all GFP_MOVABLE allocations to the CMA regions, which will free up memory
> for GFP_KERNEL allocations. 
> 
> > The idea behind the current formula is to keep cma regions free if there is
> > a plenty of other free memory, otherwise treat it on par with other memory.
> 
> With the current approach, if we have a large amount of movable memory allocated
> that has not gone into the CMA regions yet, and a DMA use case starts that
> causes the above condition to be met, we would head towards OOM conditions when
> we otherwise could have delayed this with this change.
> Note that since we're
> working on Android, there is a daemon built on top of PSI called LMKD that will
> start killing things under memory pressure (before an OOM is actually reached)
> in order to free up memory. This patch should then reduce kills accordingly for
> a better user experience by keeping a larger set of background apps alive. When
> a CMA allocation does occur and pages get migrated out, there is a similar
> reduction in headroom (you probably already know this and know of the FB
> equivalent made by Johannes Weiner).

I see... Thank you for the explanation!

So the problem is that movable allocations are spread between cma and non-cma
evenly, so that non-movable allocations might fail. And the idea is to use
the cma area more actively for movable allocations to keep a headroom for
non-movable allocations. Is it correct?

Then _maybe_ a new knob is justified, at least I don't have better ideas.
Rik, do you have any input here?

Let's then define it in a more generic way and _maybe_ move to the cma
sysfs/debugfs (not 100% sure about this part, but probably worth exploring).

Thanks!