From:   Sergey Senozhatsky <senozhatsky@chromium.org>
To:     Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Yosry Ahmed <yosryahmed@google.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Sergey Senozhatsky <senozhatsky@chromium.org>
Subject: [PATCHv3 1/4] zsmalloc: remove insert_zspage() ->inuse optimization
Date:   Fri,  3 Mar 2023 16:31:27 +0900
Message-Id: <20230303073130.1950714-2-senozhatsky@chromium.org>
In-Reply-To: <20230303073130.1950714-1-senozhatsky@chromium.org>
References: <20230303073130.1950714-1-senozhatsky@chromium.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

This optimization has no effect. It only ensures that
when a zspage was added to its corresponding fullness
list, its "inuse" counter was higher or lower than the
"inuse" counter of the zspage at the head of the list.
The intention was to keep busy zspages at the head, so
they could be filled up and moved to the ZS_FULL
fullness group more quickly. However, this doesn't work
as the "inuse" counter of a zspage can be modified by
obj_free() but the zspage may still belong to the same
fullness list. So, fix_fullness_group() won't change
the zspage's position in relation to the head's "inuse"
counter, leading to a largely random order of zspages
within the fullness list.

For instance, consider a printout of the "inuse"
counters of the first 10 zspages in a class that holds
93 objects per zspage:

 ZS_ALMOST_EMPTY:  36  67  68  64  35  54  63  52

As we can see the zspage with the lowest "inuse" counter
is actually the head of the fullness list.

Remove this pointless "optimisation".

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 mm/zsmalloc.c | 21 ++++-----------------
 1 file changed, 4 insertions(+), 17 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 3aed46ab7e6c..1cd180caff76 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -753,32 +753,19 @@ static enum fullness_group get_fullness_group(struct size_class *class,
 }
 
 /*
- * Each size class maintains various freelists and zspages are assigned
- * to one of these freelists based on the number of live objects they
- * have. This functions inserts the given zspage into the freelist
- * identified by <class, fullness_group>.
+ * This function adds the given zspage to the fullness list identified
+ * by <class, fullness_group>.
  */
 static void insert_zspage(struct size_class *class,
 				struct zspage *zspage,
 				enum fullness_group fullness)
 {
-	struct zspage *head;
-
 	class_stat_inc(class, fullness, 1);
-	head = list_first_entry_or_null(&class->fullness_list[fullness],
-					struct zspage, list);
-	/*
-	 * We want to see more ZS_FULL pages and less almost empty/full.
-	 * Put pages with higher ->inuse first.
-	 */
-	if (head && get_zspage_inuse(zspage) < get_zspage_inuse(head))
-		list_add(&zspage->list, &head->list);
-	else
-		list_add(&zspage->list, &class->fullness_list[fullness]);
+	list_add(&zspage->list, &class->fullness_list[fullness]);
 }
 
 /*
- * This function removes the given zspage from the freelist identified
+ * This function removes the given zspage from the fullness list identified
  * by <class, fullness_group>.
  */
 static void remove_zspage(struct size_class *class,
-- 
2.40.0.rc0.216.gc4246ad0f0-goog