kernel/drivers/md/bcache/extents.c, branch 0x221E-v0.0.1-v6.19

Revert "bcache: remove heap-related macros and switch to generic min_heap"

2025-06-20T03:48:03Z

This reverts commit 866898efbb25bb44fd42848318e46db9e785973a. The generic bottom-up min_heap implementation causes performance regression in invalidate_buckets_lru(), a hot path in bcache. Before the cache is fully populated, new_bucket_prio() often returns zero, leading to many equal comparisons. In such cases, bottom-up sift_down performs up to 2 * log2(n) comparisons, while the original top-down approach completes with just O() comparisons, resulting in a measurable performance gap. The performance degradation is further worsened by the non-inlined min_heap API functions introduced in commit 92a8b224b833 ("lib/min_heap: introduce non-inline versions of min heap API functions"), adding function call overhead to this critical path. As reported by Robert, bcache now suffers from latency spikes, with P100 (max) latency increasing from 600 ms to 2.4 seconds every 5 minutes. These regressions degrade bcache's effectiveness as a low-latency cache layer and lead to frequent timeouts and application stalls in production environments. This revert aims to restore bcache's original low-latency behavior. Link: https://lore.kernel.org/lkml/CAJhEC05+0S69z+3+FB2Cd0hD+pCRyWTKLEOsc8BOmH73p1m+KQ@mail.gmail.com Link: https://lkml.kernel.org/r/20250614202353.1632957-3-visitorckw@gmail.com Fixes: 866898efbb25 ("bcache: remove heap-related macros and switch to generic min_heap") Fixes: 92a8b224b833 ("lib/min_heap: introduce non-inline versions of min heap API functions") Signed-off-by: Kuan-Wei Chiu Reported-by: Robert Pang Closes: https://lore.kernel.org/linux-bcache/CAJhEC06F_AtrPgw2-7CvCqZgeStgCtitbD-ryuPpXQA-JG5XXw@mail.gmail.com Acked-by: Coly Li Cc: Ching-Chun (Jim) Huang Cc: Kent Overstreet Cc: Signed-off-by: Andrew Morton

Revert "bcache: update min_heap_callbacks to use default builtin swap"

2025-06-20T03:48:02Z

Patch series "bcache: Revert min_heap migration due to performance regression". This patch series reverts the migration of bcache from its original heap implementation to the generic min_heap library. While the original change aimed to simplify the code and improve maintainability, it introduced a severe performance regression in real-world scenarios. As reported by Robert, systems using bcache now suffer from periodic latency spikes, with P100 (max) latency increasing from 600 ms to 2.4 seconds every 5 minutes. This degrades bcache's value as a low-latency caching layer, and leads to frequent timeouts and application stalls in production environments. The primary cause of this regression is the behavior of the generic min_heap implementation's bottom-up sift_down, which performs up to 2 * log2(n) comparisons when many elements are equal. The original top-down variant used by bcache only required O(1) comparisons in such cases. The issue was further exacerbated by commit 92a8b224b833 ("lib/min_heap: introduce non-inline versions of min heap API functions"), which introduced non-inlined versions of the min_heap API, adding function call overhead to a performance-critical hot path. This patch (of 3): This reverts commit 3d8a9a1c35227c3f1b0bd132c9f0a80dbda07b65. Although removing the custom swap function simplified the code, this change is part of a broader migration to the generic min_heap API that introduced significant performance regressions in bcache. As reported by Robert, bcache now suffers from latency spikes, with P100 (max) latency increasing from 600 ms to 2.4 seconds every 5 minutes. These regressions degrade bcache's effectiveness as a low-latency cache layer and lead to frequent timeouts and application stalls in production environments. This revert is part of a series of changes to restore previous performance by undoing the min_heap transition. Link: https://lkml.kernel.org/r/20250614202353.1632957-1-visitorckw@gmail.com Link: https://lore.kernel.org/lkml/CAJhEC05+0S69z+3+FB2Cd0hD+pCRyWTKLEOsc8BOmH73p1m+KQ@mail.gmail.com Link: https://lkml.kernel.org/r/20250614202353.1632957-2-visitorckw@gmail.com Fixes: 866898efbb25 ("bcache: remove heap-related macros and switch to generic min_heap") Fixes: 92a8b224b833 ("lib/min_heap: introduce non-inline versions of min heap API functions") Signed-off-by: Kuan-Wei Chiu Reported-by: Robert Pang Closes: https://lore.kernel.org/linux-bcache/CAJhEC06F_AtrPgw2-7CvCqZgeStgCtitbD-ryuPpXQA-JG5XXw@mail.gmail.com Acked-by: Coly Li Cc: Ching-Chun (Jim) Huang Cc: Kent Overstreet Cc: Signed-off-by: Andrew Morton

bcache: update min_heap_callbacks to use default builtin swap

2024-11-06T01:12:36Z

Replace the swp function pointer in the min_heap_callbacks of bcache with NULL, allowing direct usage of the default builtin swap implementation. This modification simplifies the code and improves performance by removing unnecessary function indirection. Link: https://lkml.kernel.org/r/20241020040200.939973-8-visitorckw@gmail.com Signed-off-by: Kuan-Wei Chiu Cc: Adrian Hunter Cc: Arnaldo Carvalho de Melo Cc: Ching-Chun (Jim) Huang Cc: Coly Li Cc: Ian Rogers Cc: Ingo Molnar Cc: Jiri Olsa Cc: Jonathan Corbet Cc: Kent Overstreet Cc: "Liang, Kan" Cc: Mark Rutland Cc: Matthew Sakai Cc: Matthew Wilcox (Oracle) Cc: Namhyung Kim Cc: Peter Zijlstra Signed-off-by: Andrew Morton

bcache: remove heap-related macros and switch to generic min_heap

2024-06-25T05:25:00Z

Drop the heap-related macros from bcache and replacing them with the generic min_heap implementation from include/linux. By doing so, code readability is improved by using functions instead of macros. Moreover, the min_heap implementation in include/linux adopts a bottom-up variation compared to the textbook version currently used in bcache. This bottom-up variation allows for approximately 50% reduction in the number of comparison operations during heap siftdown, without changing the number of swaps, thus making it more efficient. Link: https://lkml.kernel.org/ioyfizrzq7w7mjrqcadtzsfgpuntowtjdw5pgn4qhvsdp4mqqg@nrlek5vmisbu Link: https://lkml.kernel.org/r/20240524152958.919343-16-visitorckw@gmail.com Signed-off-by: Kuan-Wei Chiu Reviewed-by: Ian Rogers Acked-by: Coly Li Cc: Adrian Hunter Cc: Alexander Shishkin Cc: Arnaldo Carvalho de Melo Cc: Bagas Sanjaya Cc: Brian Foster Cc: Ching-Chun (Jim) Huang Cc: Ingo Molnar Cc: Jiri Olsa Cc: Kent Overstreet Cc: Mark Rutland Cc: Matthew Sakai Cc: Namhyung Kim Cc: Peter Zijlstra Cc: Randy Dunlap Signed-off-by: Andrew Morton

bcache: remove PTR_CACHE

2021-04-11T14:37:55Z

Remove the PTR_CACHE inline and replace it with a direct dereference of c->cache. (Coly Li: fix the typo from PTR_BUCKET to PTR_CACHE in commit log) Signed-off-by: Christoph Hellwig Signed-off-by: Coly Li Link: https://lore.kernel.org/r/20210411134316.80274-3-colyli@suse.de Signed-off-by: Jens Axboe

bcache: remove embedded struct cache_sb from struct cache_set

2020-10-02T20:25:30Z

Since bcache code was merged into mainline kerrnel, each cache set only as one single cache in it. The multiple caches framework is here but the code is far from completed. Considering the multiple copies of cached data can also be stored on e.g. md raid1 devices, it is unnecessary to support multiple caches in one cache set indeed. The previous preparation patches fix the dependencies of explicitly making a cache set only have single cache. Now we don't have to maintain an embedded partial super block in struct cache_set, the in-memory super block can be directly referenced from struct cache. This patch removes the embedded struct cache_sb from struct cache_set, and fixes all locations where the superb lock was referenced from this removed super block by referencing the in-memory super block of struct cache. Signed-off-by: Coly Li Reviewed-by: Hannes Reinecke Signed-off-by: Jens Axboe

bcache: Convert pr_ uses to a more typical style

2020-05-27T11:19:36Z

Remove the trailing newline from the define of pr_fmt and add newlines to the uses. Miscellanea: o Convert bch_bkey_dump from multiple uses of pr_err to pr_cont as the earlier conversion was inappropriate done causing multiple lines to be emitted where only a single output line was desired o Use vsprintf extension %pV in bch_cache_set_error to avoid multiple line output where only a single line output was desired o Coalesce formats Fixes: 6ae63e3501c4 ("bcache: replace printk() by pr_*() routines") Signed-off-by: Joe Perches Signed-off-by: Coly Li Signed-off-by: Jens Axboe

bcache: treat stale && dirty keys as bad keys

2019-02-09T14:18:31Z

Stale && dirty keys can be produced in the follow way: After writeback in write_dirty_finish(), dirty keys k1 will replace by clean keys k2 ==>ret = bch_btree_insert(dc->disk.c, &keys, NULL, &w->key); ==>btree_insert_fn(struct btree_op *b_op, struct btree *b) ==>static int bch_btree_insert_node(struct btree *b, struct btree_op *op, struct keylist *insert_keys, atomic_t *journal_ref, Then two steps: A) update k1 to k2 in btree node memory; bch_btree_insert_keys(b, op, insert_keys, replace_key) B) Write the bset(contains k2) to cache disk by a 30s delay work bch_btree_leaf_dirty(b, journal_ref). But before the 30s delay work write the bset to cache device, these things happened: A) GC works, and reclaim the bucket k2 point to; B) Allocator works, and invalidate the bucket k2 point to, and increase the gen of the bucket, and place it into free_inc fifo; C) Until now, the 30s delay work still does not finish work, so in the disk, the key still is k1, it is dirty and stale (its gen is smaller than the gen of the bucket). and then the machine power off suddenly happens; D) When the machine power on again, after the btree reconstruction, the stale dirty key appear. In bch_extent_bad(), when expensive_debug_checks is off, it would treat the dirty key as good even it is stale keys, and it would cause bellow probelms: A) In read_dirty() it would cause machine crash: BUG_ON(ptr_stale(dc->disk.c, &w->key, 0)); B) It could be worse when reads hits stale dirty keys, it would read old incorrect data. This patch tolerate the existence of these stale && dirty keys, and treat them as bad key in bch_extent_bad(). (Coly Li: fix indent which was modified by sender's email client) Signed-off-by: Tang Junhui Cc: stable@vger.kernel.org Signed-off-by: Coly Li Signed-off-by: Jens Axboe

bcache: replace hard coded number with BUCKET_GC_GEN_MAX

2018-10-08T14:19:55Z

In extents.c:bch_extent_bad(), number 96 is used as parameter to call btree_bug_on(). The purpose is to check whether stale gen value exceeds BUCKET_GC_GEN_MAX, so it is better to use macro BUCKET_GC_GEN_MAX to make the code more understandable. Signed-off-by: Coly Li Signed-off-by: Jens Axboe

bcache: replace printk() by pr_*() routines

2018-08-11T21:46:41Z

There are still many places in bcache use printk to display kernel message, which are suggested to be preplaced by pr_*() routines like pr_err(), pr_info(), or pr_notice(). This patch replaces all printk() with a proper pr_*() routine for bcache code. Signed-off-by: Coly Li Reviewed-by: Shenghui Wang Signed-off-by: Jens Axboe