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

[Feng Tang]

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?

Thanks,
Feng

> > @@ -1931,6 +1954,7 @@ unsigned int mempolicy_slab_node(void)
> >  
> >  	switch (policy->mode) {
> >  	case MPOL_PREFERRED:
> > +	case MPOL_PREFERRED_MANY:
> >  		return first_node(policy->nodes);
> 
> Similarly here but I am not really familiar with the slab numa code
> enough to have strong opinions here.
> 
> > @@ -2173,10 +2198,12 @@ struct page *alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
> >  		 * node and don't fall back to other nodes, as the cost of
> >  		 * remote accesses would likely offset THP benefits.
> >  		 *
> > -		 * If the policy is interleave, or does not allow the current
> > -		 * node in its nodemask, we allocate the standard way.
> > +		 * If the policy is interleave or multiple preferred nodes, or
> > +		 * does not allow the current node in its nodemask, we allocate
> > +		 * the standard way.
> >  		 */
> > -		if (pol->mode == MPOL_PREFERRED)
> > +		if ((pol->mode == MPOL_PREFERRED ||
> > +		     pol->mode == MPOL_PREFERRED_MANY))
> >  			hpage_node = first_node(pol->nodes);
> 
> Same here.
> 
> > @@ -2451,6 +2479,9 @@ int mpol_misplaced(struct page *page, struct vm_area_struct *vma, unsigned long
> >  		break;
> >  
> >  	case MPOL_PREFERRED:
> > +	case MPOL_PREFERRED_MANY:
> > +		if (node_isset(curnid, pol->nodes))
> > +			goto out;
> >  		polnid = first_node(pol->nodes);
> >  		break;
> 
> I do not follow what is the point of using first_node here. Either the
> node is in the mask or it is misplaced. What are you trying to achieve
> here?
> -- 
> Michal Hocko
> SUSE Labs