Re: [PATCH 00/18] multiple preferred nodes

On 20-06-24 09:52:16, Michal Hocko wrote:

On Tue 23-06-20 09:12:11, Ben Widawsky wrote:

quoted

On 20-06-23 13:20:48, Michal Hocko wrote:

[...]

quoted

quoted

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?

Hi Michal,

I'm mostly done implementing this change. It looks good, and so far I think it's
functionally equivalent. One thing though, above you use NULL for the fallback.
That actually should not be NULL because of the logic in policy_node to restrict
zones, and obey cpusets. I've implemented it as such, but I was hoping someone
with a deeper understanding, and more experience can confirm that was the
correct thing to do.

Thanks.