Re: [PATCH 4/9] mm, hugetlb: fix and clean-up node iteration code to alloc or free

Joonsoo Kim
2013-07-16T01:41:21Z

On Mon, Jul 15, 2013 at 07:57:37PM +0530, Aneesh Kumar K.V wrote:
> Joonsoo Kim <iamjoonsoo.kim@lge.com> writes:
>
> > Current node iteration code have a minor problem which do one more
> > node rotation if we can't succeed to allocate. For example,
> > if we start to allocate at node 0, we stop to iterate at node 0.
> > Then we start to allocate at node 1 for next allocation.
>
> Can you explain the problem in a bit more detail

Yes.
I try to explain with below example.

Assume that we start with below condition.

h->next_nid_to_alloc = 0;
node_allowes = 0, 1;
and think we are in alloc_fresh_huge_page().

So, start to execute.

start_nid = 0;
next_nid = 0;
h->next_nid_to_alloc = 1;
And then go into the loop.

We try to allocate from node 0.
and if failed, call hstate_next_node_to_alloc() to get next node.
So,
next_nid = 1;
h->next_nid_to_alloc = 0;
And then try to allocate from node 1.

If we fail again,
next_nid = 0;
h->next_nid_to_alloc = 1;

And next_nid is same as start_nid, so we quit the loop.
If we do alloc_fresh_huge_page() next time, we try to allocate
from *not node 0*, but *node 1*.

This is the problem I mentioned.

>
> >
> > I introduce new macros "for_each_node_mask_to_[alloc|free]" and
> > fix and clean-up node iteration code to alloc or free.
> > This makes code more understandable.
> >
>
> I found the existing code more readable. Obviously I haven't yet figured
> out the problem you have observed with the code.
>
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 0067cf4..a838e6b 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -752,33 +752,6 @@ static int hstate_next_node_to_alloc(struct hstate *h,
> > return nid;
> > }
> >
> > -static int alloc_fresh_huge_page(struct hstate *h, nodemask_t *nodes_allowed)
> > -{
> > - struct page *page;
> > - int start_nid;
> > - int next_nid;
> > - int ret = 0;
> > -
> > - start_nid = hstate_next_node_to_alloc(h, nodes_allowed);
> > - next_nid = start_nid;
> > -
> > - do {
> > - page = alloc_fresh_huge_page_node(h, next_nid);
> > - if (page) {
> > - ret = 1;
> > - break;
> > - }
> > - next_nid = hstate_next_node_to_alloc(h, nodes_allowed);
> > - } while (next_nid != start_nid);
> > -
> > - if (ret)
> > - count_vm_event(HTLB_BUDDY_PGALLOC);
> > - else
> > - count_vm_event(HTLB_BUDDY_PGALLOC_FAIL);
> > -
> > - return ret;
> > -}
> > -
> > /*
> > * helper for free_pool_huge_page() - return the previously saved
> > * node ["this node"] from which to free a huge page. Advance the
> > @@ -797,6 +770,42 @@ static int hstate_next_node_to_free(struct hstate *h, nodemask_t *nodes_allowed)
> > return nid;
> > }
> >
> > +#define for_each_node_mask_to_alloc(hs, nr_nodes, node, mask) \
> > + for (nr_nodes = nodes_weight(*mask), \
> > + node = hstate_next_node_to_alloc(hs, mask); \
> > + nr_nodes > 0 && \
> > + ((node = hstate_next_node_to_alloc(hs, mask)) || 1); \
> > + nr_nodes--)
> > +
> > +#define for_each_node_mask_to_free(hs, nr_nodes, node, mask) \
> > + for (nr_nodes = nodes_weight(*mask), \
> > + node = hstate_next_node_to_free(hs, mask); \
> > + nr_nodes > 0 && \
> > + ((node = hstate_next_node_to_free(hs, mask)) || 1); \
> > + nr_nodes--)
> > +
> > +static int alloc_fresh_huge_page(struct hstate *h, nodemask_t *nodes_allowed)
> > +{
> > + struct page *page;
> > + int nr_nodes, node;
> > + int ret = 0;
> > +
> > + for_each_node_mask_to_alloc(h, nr_nodes, node, nodes_allowed) {
>
>
> This check for nodes_weight and fail right ? (nr_nodes == 0). That is
> not the case with the existing code. It will allocate from
> h->next_nid_to_alloc. Is that ok ?

Above code also try to allocate from h->next_nid_to_alloc.
nr_nodes is used for determining when we quit the loop.
Above code means that if all nodes in node_mask are traversed, quit the loop.

Thanks.

>
>
> > + page = alloc_fresh_huge_page_node(h, node);
> > + if (page) {
> > + ret = 1;
> > + break;
> > + }
> > + }
> > +
> > + if (ret)
> > + count_vm_event(HTLB_BUDDY_PGALLOC);
> > + else
> > + count_vm_event(HTLB_BUDDY_PGALLOC_FAIL);
> > +
> > + return ret;
> > +}
> > +
>
> -aneesh
>
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