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SubjectRe: [PATCH] mm: Optimize put_mems_allowed() usage
On Tue, 27 Mar 2012 15:14:30 +0200
Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> Subject: mm: Optimize put_mems_allowed() usage
> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date: Mon Mar 26 14:13:05 CEST 2012
>
> Since put_mems_allowed() is strictly optional, its a seqcount retry,
> we don't need to evaluate the function if the allocation was in fact
> successful, saving a smp_rmb some loads and comparisons on some
> relative fast-paths.
>
> Since the naming, get/put_mems_allowed() does suggest a mandatory
> pairing, rename the interface, as suggested by Mel, to resemble the
> seqcount interface.
>
> This gives us: read_mems_allowed_begin() and
> read_mems_allowed_retry(), where it is important to note that the
> return value of the latter call is inverted from its previous
> incarnation.
>
> ...
>
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1606,7 +1606,7 @@ static struct page *get_any_partial(stru
> return NULL;
>
> do {
> - cpuset_mems_cookie = get_mems_allowed();
> + cpuset_mems_cookie = read_mems_allowed_begin();
> zonelist = node_zonelist(slab_node(current->mempolicy), flags);
> for_each_zone_zonelist(zone, z, zonelist, high_zoneidx) {
> struct kmem_cache_node *n;
> @@ -1616,21 +1616,11 @@ static struct page *get_any_partial(stru
> if (n && cpuset_zone_allowed_hardwall(zone, flags) &&
> n->nr_partial > s->min_partial) {
> object = get_partial_node(s, n, c);
> - if (object) {
> - /*
> - * Return the object even if
> - * put_mems_allowed indicated that
> - * the cpuset mems_allowed was
> - * updated in parallel. It's a
> - * harmless race between the alloc
> - * and the cpuset update.
> - */
> - put_mems_allowed(cpuset_mems_cookie);
> + if (object)
> return object;
> - }
> }
> }
> - } while (!put_mems_allowed(cpuset_mems_cookie));
> + } while (read_mems_allowed_retry(cpuset_mems_cookie));

I do think it was a bad idea to remove that comment. As it stands, the
reader will be wondering why we did the read_mems_allowed_begin() at
all, and whether failing to check for a change is a bug.

--- a/mm/slub.c~mm-optimize-put_mems_allowed-usage-fix
+++ a/mm/slub.c
@@ -1624,8 +1624,16 @@ static struct page *get_any_partial(stru
if (n && cpuset_zone_allowed_hardwall(zone, flags) &&
n->nr_partial > s->min_partial) {
object = get_partial_node(s, n, c);
- if (object)
+ if (object) {
+ /*
+ * Don't check read_mems_allowed_retry()
+ * here - if mems_allowed was updated in
+ * parallel, that was a harmless race
+ * between allocation and the cpuset
+ * update
+ */
return object;
+ }
}
}
} while (read_mems_allowed_retry(cpuset_mems_cookie));
_


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