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| author | Johannes Weiner <hannes@cmpxchg.org> | 2021-02-25 17:16:51 -0800 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2021-02-26 09:41:00 -0800 |
| commit | 629484ae73754243917e06d8d5e5f37c26e99399 (patch) | |
| tree | c3d5422b5127aea32f8d73b2909dd66673cd2213 | |
| parent | 2bbd00aef0671bfe3c2ca5ba67097246257de125 (diff) | |
| download | linux-leds-629484ae73754243917e06d8d5e5f37c26e99399.tar.gz | |
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>
| -rw-r--r-- | include/linux/vmstat.h | 6 | ||||
| -rw-r--r-- | mm/vmstat.c | 12 |
2 files changed, 18 insertions, 0 deletions
diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h index 773135fc6e19c7..506d625163a11c 100644 --- a/include/linux/vmstat.h +++ b/include/linux/vmstat.h @@ -313,6 +313,12 @@ static inline void __mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item, int delta) { 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; } diff --git a/mm/vmstat.c b/mm/vmstat.c index 0b0fc3b77789b9..e60b36f5f0a9ee 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; } |