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authorJohannes Weiner <hannes@cmpxchg.org>2021-02-25 17:16:51 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2021-02-26 09:41:00 -0800
commit629484ae73754243917e06d8d5e5f37c26e99399 (patch)
treec3d5422b5127aea32f8d73b2909dd66673cd2213 /mm/vmstat.c
parent2bbd00aef0671bfe3c2ca5ba67097246257de125 (diff)
mm: vmstat: add some comments on internal storage of byte items
Byte-accounted items are used for slab object accounting at the cgroup level, because the objects in a slab page can belong to different cgroups. At the global level these items always change in multiples of whole slab pages. The vmstat code exploits this and stores these items as pages internally, which allows for more compact per-cpu data. This optimization isn't self-evident from the asserts and the division in the stat update functions. Provide the reader with some context. Link: https://lkml.kernel.org/r/20210202184411.118614-1-hannes@cmpxchg.org Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/vmstat.c')
-rw-r--r--mm/vmstat.c12
1 files changed, 12 insertions, 0 deletions
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 0b0fc3b77789..e60b36f5f0a9 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -342,6 +342,12 @@ void __mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
long t;
if (vmstat_item_in_bytes(item)) {
+ /*
+ * Only cgroups use subpage accounting right now; at
+ * the global level, these items still change in
+ * multiples of whole pages. Store them as pages
+ * internally to keep the per-cpu counters compact.
+ */
VM_WARN_ON_ONCE(delta & (PAGE_SIZE - 1));
delta >>= PAGE_SHIFT;
}
@@ -551,6 +557,12 @@ static inline void mod_node_state(struct pglist_data *pgdat,
long o, n, t, z;
if (vmstat_item_in_bytes(item)) {
+ /*
+ * Only cgroups use subpage accounting right now; at
+ * the global level, these items still change in
+ * multiples of whole pages. Store them as pages
+ * internally to keep the per-cpu counters compact.
+ */
VM_WARN_ON_ONCE(delta & (PAGE_SIZE - 1));
delta >>= PAGE_SHIFT;
}