Re: [RFC 0/4] RFC - Coherent Device Memory (Not for inclusion)

[Michal Hocko <mhocko@suse.com>]

On Thu 04-05-17 15:26:55, Balbir Singh wrote:
> On Tue, 2017-05-02 at 16:36 +0200, Michal Hocko wrote:
> > On Wed 19-04-17 17:52:38, Balbir Singh wrote:
[...]
> > > 2. kswapd reclaim
> > 
> > How is the memory reclaim handled then? How are users expected to handle
> > OOM situation?
> > 
> 
> 1. The fallback node list for coherent memory includes regular memory
>    nodes
> 2. Direct reclaim works, I've tested it

But the direct reclaim would be effective only _after_ all other nodes
are full.

I thought that kswapd reclaim is a problem because the HW doesn't
support aging properly but as the direct reclaim works then what is the
actual problem?
 
> > > The reason for exposing this device memory as NUMA is to simplify
> > > the programming model, where memory allocation via malloc() or
> > > mmap() for example would seamlessly work across both kinds of
> > > memory. Since we expect the size of device memory to be smaller
> > > than system RAM, we would like to control the allocation of such
> > > memory. The proposed mechanism reuses nodemasks and explicit
> > > specification of the coherent node in the nodemask for allocation
> > > from device memory. This implementation also allows for kernel
> > > level allocation via __GFP_THISNODE and existing techniques
> > > such as page migration to work.
> > 
> > so it basically resembles isol_cpus except for memory, right. I believe
> > scheduler people are more than unhappy about this interface...
> >
> 
> isol_cpus were for an era when timer/interrupts and other scheduler
> infrastructure present today was not around, but I don't mean to digress.

AFAIU, it has been added to _isolate_ some cpus from the scheduling domain
and have them available for the explicit affinity usage. You are
effectivelly proposing the same thing.

-- 
Michal Hocko
SUSE Labs

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