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

[Aneesh Kumar K V]

On 4/25/22 7:27 PM, Jonathan Cameron wrote:
> On Mon, 25 Apr 2022 16:45:38 +0530
> Jagdish Gediya <jvgediya@xxxxxxxxxxxxx> 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?



-aneesh