Re: [PATCH v3 0/7] mm: demotion: Introduce new node state N_DEMOTION_TARGETS

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Mon, 25 Apr 2022 20:23:56 +0530
Aneesh Kumar K V <aneesh.kumar@linux.ibm.com> wrote:

> On 4/25/22 7:27 PM, Jonathan Cameron wrote:
> > On Mon, 25 Apr 2022 16:45:38 +0530
> > Jagdish Gediya <jvgediya@linux.ibm.com> wrote:
> >  
> 
> ....
> 
> >> $ numactl -H
> >> available: 2 nodes (0-1)
> >> node 0 cpus: 0 1 2 3 4 5 6 7
> >> node 0 size: 14272 MB
> >> node 0 free: 13392 MB
> >> node 1 cpus:
> >> node 1 size: 2028 MB
> >> node 1 free: 1971 MB
> >> node distances:
> >> node   0   1
> >>    0:  10  40
> >>    1:  40  10
> >>
> >> 1) without N_DEMOTION_TARGETS patch series, 1 is demotion target
> >>     for 0 even when 1 is DRAM node and there is no demotion targets for 1.  
> > 
> > I'm not convinced the distinction between DRAM and persistent memory is
> > valid. There will definitely be systems with a large pool
> > of remote DRAM (and potentially no NV memory) where the right choice
> > is to demote to that DRAM pool.
> > 
> > Basing the decision on whether the memory is from kmem or
> > normal DRAM doesn't provide sufficient information to make the decision.
> >   
> >>
> >> $ cat /sys/bus/nd/devices/dax0.0/target_node
> >> 2
> >> $
> >> # cd /sys/bus/dax/drivers/
> >> :/sys/bus/dax/drivers# ls
> >> device_dax  kmem
> >> :/sys/bus/dax/drivers# cd device_dax/
> >> :/sys/bus/dax/drivers/device_dax# echo dax0.0 > unbind
> >> :/sys/bus/dax/drivers/device_dax# echo dax0.0 >  ../kmem/new_id
> >> :/sys/bus/dax/drivers/device_dax# numactl -H
> >> available: 3 nodes (0-2)
> >> node 0 cpus: 0 1 2 3 4 5 6 7
> >> node 0 size: 14272 MB
> >> node 0 free: 13380 MB
> >> node 1 cpus:
> >> node 1 size: 2028 MB
> >> node 1 free: 1961 MB
> >> node 2 cpus:
> >> node 2 size: 0 MB
> >> node 2 free: 0 MB
> >> node distances:
> >> node   0   1   2
> >>    0:  10  40  80
> >>    1:  40  10  80
> >>    2:  80  80  10
> >>
> >> 2) Once this new node brought online,  without N_DEMOTION_TARGETS
> >> patch series, 1 is demotion target for 0 and 2 is demotion target
> >> for 1.
> >>
> >> With this patch series applied,
> >> 1) No demotion target for either 0 or 1 before dax device is online  
> > 
> > I'd argue that is wrong.  At this state you have a tiered memory system
> > be it one with just DRAM.  Using it as such is correct behavior that
> > we should not be preventing.  Sure some usecases wouldn't want that
> > arrangement but some do want it.
> >   
> 
> I missed this in my earlier reply. Are you suggesting that we would want 
> Node 1 (DRAM only memory numa node) to act as demotion target for Node 
> 0?  Any reason why we would want to do that? That is clearly opposite of 
> what we are trying to do here. IMHO node using Node1 as demotion target 
> for Node0 is a better default?

In this case, because of the small size that probably wouldn't make sense.
But, if that were a CXL memory pool with multiple TB of DDR then yes
we would want the default case to use that memory for the demotion path.

So I don't think DDR vs NV via kmem alone is the right basis for a decision
on the default behavior.

Sure we can make this all a userspace problem.

Jonathan

> 
> 
> 
> -aneesh