Re: [PATCH v6 1/6] mm/mempolicy: Add MPOL_PREFERRED_MANY for multiple preferred nodes

[Michal Hocko <mhocko@suse.com>]

On Thu 29-07-21 15:09:18, Feng Tang wrote:
> On Wed, Jul 28, 2021 at 06:12:21PM +0200, Michal Hocko wrote:
> > On Wed 28-07-21 22:11:56, Feng Tang wrote:
> > > On Wed, Jul 28, 2021 at 02:31:03PM +0200, Michal Hocko wrote:
> > > > [Sorry for a late review]
> > > 
> > > Not at all. Thank you for all your reviews and suggestions from v1
> > > to v6!
> > > 
> > > > On Mon 12-07-21 16:09:29, Feng Tang wrote:
> > > > [...]
> > > > > @@ -1887,7 +1909,8 @@ nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *policy)
> > > > >  /* Return the node id preferred by the given mempolicy, or the given id */
> > > > >  static int policy_node(gfp_t gfp, struct mempolicy *policy, int nd)
> > > > >  {
> > > > > -	if (policy->mode == MPOL_PREFERRED) {
> > > > > +	if (policy->mode == MPOL_PREFERRED ||
> > > > > +	    policy->mode == MPOL_PREFERRED_MANY) {
> > > > >  		nd = first_node(policy->nodes);
> > > > >  	} else {
> > > > >  		/*
> > > > 
> > > > Do we really want to have the preferred node to be always the first node
> > > > in the node mask? Shouldn't that strive for a locality as well? Existing
> > > > callers already prefer numa_node_id() - aka local node - and I belive we
> > > > shouldn't just throw that away here.
> > >  
> > > I think it's about the difference of 'local' and 'prefer/perfer-many'
> > > policy. There are different kinds of memory HW: HBM(High Bandwidth
> > > Memory), normal DRAM, PMEM (Persistent Memory), which have different
> > > price, bandwidth, speed etc. A platform may have two, or all three of
> > > these types, and there are real use case which want memory comes
> > > 'preferred' node/nodes than the local node.
> > > 
> > > And good point for 'local node', if the 'prefer-many' policy's
> > > nodemask has local node set, we should pick it han this
> > > 'first_node', and the same semantic also applies to the other
> > > several places you pointed out. Or do I misunderstand you point?
> > 
> > Yeah. Essentially what I am trying to tell is that for
> > MPOL_PREFERRED_MANY you simply want to return the given node without any
> > alternation. That node will be used for the fallback zonelist and the
> > nodemask would make sure we won't get out of the policy.
> 
> I think I got your point now :)
> 
> With current mainline code, the 'prefer' policy will return the preferred
> node.

Yes this makes sense as there is only one node.

> For 'prefer-many', we would like to keep the similar semantic, that the
> preference of node is 'preferred' > 'local' > all other nodes.

Yes but which of the preferred nodes you want to start with. Say your
nodemask preferring nodes 0 and 2 with the following topology
	0	1	2	3
0	10	30	20	30
1	30	10	20	30
2	20	30	10	30
3	30	30	30	10

And say you are running on cpu 1. I believe you want your allocation
preferably from node 2 rathern than 0, right? With your approach you
would start with node 0 which would be more distant from cpu 1. Also the
semantic to give nodes some ordering based on their numbers sounds
rather weird to me.

The semantic I am proposing is to allocate from prefered nodes in
distance order starting from the local node.
-- 
Michal Hocko
SUSE Labs