mm: swap-token: add a comment for priority aging

Document some swap token aging design decisions.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/mm/thrash.c b/mm/thrash.c
index 42ffb0179271..e53f7d02c17c 100644
--- a/mm/thrash.c
+++ b/mm/thrash.c
@@ -67,6 +67,17 @@ void grab_swap_token(struct mm_struct *mm)
         if (!swap_token_mm)
                 goto replace_token;
 
+        /*
+         * Usually, we don't need priority aging because long interval faults
+         * makes priority decrease quickly. But there is one exception. If the
+         * token owner task is sleeping, it never make long interval faults.
+         * Thus, we need a priority aging mechanism instead. The requirements
+         * of priority aging are
+         *  1) An aging interval is reasonable enough long. Too short aging
+         *     interval makes quick swap token lost and decrease performance.
+         *  2) The swap token owner task have to get priority aging even if
+         *     it's under sleep.
+         */
         if ((global_faults - last_aging) > TOKEN_AGING_INTERVAL) {
                 swap_token_mm->token_priority /= 2;
                 last_aging = global_faults;