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

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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>
This commit is contained in:
Kuan-Wei Chiu
2026-03-20 18:09:37 +00:00
committed by Andrew Morton
parent af53e85ef7
commit 237213776d
2 changed files with 0 additions and 3 deletions
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2
fs/ubifs/gc.c
View File

@@ -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;

1
fs/ubifs/replay.c
View File

@@ -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;