On 20-06-24 21:51:58, Michal Hocko wrote:
> On Wed 24-06-20 12:37:33, Ben Widawsky wrote:
> > On 20-06-24 20:39:17, Michal Hocko wrote:
> > > 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.
> >
> > Ah, gotcha. Enabling independent masks seemed useful. Some bad decisions got me
> > to that point. UAPI cannot get independent masks, and callers of these functions
> > don't yet use them.
> >
> > So let me ask before I actually type it up and find it's much much simpler, is
> > there not some perceived benefit to having both masks being independent?
>
> I am not sure I follow. Which two masks do you have in mind? zonelist
> and user provided nodemask?
Internally, a nodemask_t for preferred node, and a nodemask_t for bound nodes.
>
> --
> Michal Hocko
> SUSE Labs
>
