On Wed 24-06-20 09:16:43, Ben Widawsky wrote:
> On 20-06-24 09:52:16, Michal Hocko wrote:
> > On Tue 23-06-20 09:12:11, Ben Widawsky wrote:
> > > On 20-06-23 13:20:48, Michal Hocko wrote:
> > [...]
> > > > It would be also great to provide a high level semantic description
> > > > here. I have very quickly glanced through patches and they are not
> > > > really trivial to follow with many incremental steps so the higher level
> > > > intention is lost easily.
> > > >
> > > > Do I get it right that the default semantic is essentially
> > > > - allocate page from the given nodemask (with __GFP_RETRY_MAYFAIL
> > > > semantic)
> > > > - fallback to numa unrestricted allocation with the default
> > > > numa policy on the failure
> > > >
> > > > Or are there any usecases to modify how hard to keep the preference over
> > > > the fallback?
> > >
> > > tl;dr is: yes, and no usecases.
> >
> > OK, then I am wondering why the change has to be so involved. Except for
> > syscall plumbing the only real change to the allocator path would be
> > something like
> >
> > static nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *policy)
> > {
> > /* Lower zones don't get a nodemask applied for MPOL_BIND */
> > if (unlikely(policy->mode == MPOL_BIND ||
> > policy->mode == MPOL_PREFERED_MANY) &&
> > apply_policy_zone(policy, gfp_zone(gfp)) &&
> > cpuset_nodemask_valid_mems_allowed(&policy->v.nodes))
> > return &policy->v.nodes;
> >
> > return NULL;
> > }
> >
> > alloc_pages_current
> >
> > if (pol->mode == MPOL_INTERLEAVE)
> > page = alloc_page_interleave(gfp, order, interleave_nodes(pol));
> > else {
> > gfp_t gfp_attempt = gfp;
> >
> > /*
> > * Make sure the first allocation attempt will try hard
> > * but eventually fail without OOM killer or other
> > * disruption before falling back to the full nodemask
> > */
> > if (pol->mode == MPOL_PREFERED_MANY)
> > gfp_attempt |= __GFP_RETRY_MAYFAIL;
> >
> > page = __alloc_pages_nodemask(gfp_attempt, order,
> > policy_node(gfp, pol, numa_node_id()),
> > policy_nodemask(gfp, pol));
> > if (!page && pol->mode == MPOL_PREFERED_MANY)
> > page = __alloc_pages_nodemask(gfp, order,
> > numa_node_id(), NULL);
> > }
> >
> > return page;
> >
> > similar (well slightly more hairy) in alloc_pages_vma
> >
> > Or do I miss something that really requires more involved approach like
> > building custom zonelists and other larger changes to the allocator?
>
> I think I'm missing how this allows selecting from multiple preferred nodes. In
> this case when you try to get the page from the freelist, you'll get the
> zonelist of the preferred node, and when you actually scan through on page
> allocation, you have no way to filter out the non-preferred nodes. I think the
> plumbing of multiple nodes has to go all the way through
> __alloc_pages_nodemask(). But it's possible I've missed the point.
policy_nodemask() will provide the nodemask which will be used as a
filter on the policy_node.
--
Michal Hocko
SUSE Labs
