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?
--
Michal Hocko
SUSE Labs
