ubifs: remove unnecessary cond_resched() from list_sort() compare

Patch series "lib/list_sort: Clean up list_sort() scheduling workarounds",
v3.

Historically, list_sort() included a hack in merge_final() that
periodically invoked dummy cmp(priv, b, b) calls when merging highly
unbalanced lists.  This allowed the caller to invoke cond_resched() within
their comparison callbacks to avoid soft lockups.

However, an audit of the kernel tree shows that fs/ubifs/ has been the
sole user of this mechanism.  For all other generic list_sort() users,
this results in wasted function calls and unnecessary overhead in a tight
loop.

Recent discussions and code inspection confirmed that the lists being
sorted in UBIFS are bounded in size (a few thousand elements at most), and
the comparison functions are extremely lightweight.  Therefore, UBIFS does
not actually need to rely on this mechanism.

This patch (of 2):

Historically, UBIFS embedded cond_resched() calls inside its list_sort()
comparison callbacks (data_nodes_cmp, nondata_nodes_cmp, and
replay_entries_cmp) to prevent soft lockups when sorting long lists.

However, further inspection by Richard Weinberger reveals that these
compare functions are extremely lightweight and do not perform any
blocking MTD I/O. Furthermore, the lists being sorted are strictly
bounded in size:
- In the GC case, the list contains at most the number of nodes that
  fit into a single LEB.
- In the replay case, the list spans across a few LEBs from the UBIFS
  journal, amounting to at most a few thousand elements.

Since the compare functions are called a few thousand times at most, the
overhead of frequent scheduling points is unjustified.  Removing the
cond_resched() calls simplifies the comparison logic and reduces
unnecessary context switch checks during the sort.

Link: https://lkml.kernel.org/r/20260320180938.1827148-1-visitorckw@gmail.com
Link: https://lkml.kernel.org/r/20260320180938.1827148-2-visitorckw@gmail.com
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Acked-by: Richard Weinberger <richard@nod.at>
Cc: Ching-Chun (Jim) Huang <jserv@ccns.ncku.edu.tw>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Mars Cheng <marscheng@google.com>
Cc: Yu-Chun Lin <eleanor15x@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/ubifs/gc.c b/fs/ubifs/gc.c
index 0bf08b7755b83c966e755dea7ec3a6ad83e30a88..933c79b5cd6b95257bb29a9edee24ce64082795f 100644 (file)
--- a/fs/ubifs/gc.c
+++ b/fs/ubifs/gc.c
@@ -109,7 +109,6 @@ static int data_nodes_cmp(void *priv, const struct list_head *a,
        struct ubifs_info *c = priv;
        struct ubifs_scan_node *sa, *sb;
 
-       cond_resched();
        if (a == b)
                return 0;
 
@@ -153,7 +152,6 @@ static int nondata_nodes_cmp(void *priv, const struct list_head *a,
        struct ubifs_info *c = priv;
        struct ubifs_scan_node *sa, *sb;
 
-       cond_resched();
        if (a == b)
                return 0;
 

diff --git a/fs/ubifs/replay.c b/fs/ubifs/replay.c
index a9a568f4a868aa727b619e70b9c22caf891cdf6f..263045e05cf18564d42f2bbcd5bae36997163a5e 100644 (file)
--- a/fs/ubifs/replay.c
+++ b/fs/ubifs/replay.c
@@ -305,7 +305,6 @@ static int replay_entries_cmp(void *priv, const struct list_head *a,
        struct ubifs_info *c = priv;
        struct replay_entry *ra, *rb;
 
-       cond_resched();
        if (a == b)
                return 0;