kernel/drivers/md/bcache/bset.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: fix variable length array abuse in btree_iter

2024-05-09T01:15:01Z

btree_iter is used in two ways: either allocated on the stack with a fixed size MAX_BSETS, or from a mempool with a dynamic size based on the specific cache set. Previously, the struct had a fixed-length array of size MAX_BSETS which was indexed out-of-bounds for the dynamically-sized iterators, which causes UBSAN to complain. This patch uses the same approach as in bcachefs's sort_iter and splits the iterator into a btree_iter with a flexible array member and a btree_iter_stack which embeds a btree_iter as well as a fixed-length data array. Cc: stable@vger.kernel.org Closes: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2039368 Signed-off-by: Matthew Mirvish Signed-off-by: Coly Li Link: https://lore.kernel.org/r/20240509011117.2697-3-colyli@suse.de Signed-off-by: Jens Axboe

bcache: bset: Fix comment typos

2022-09-19T17:12:35Z

Remove the redundant word `by`, correct the typo `creaated`. CC: Kent Overstreet CC: linux-bcache@vger.kernel.org Signed-off-by: Jules Maselbas Signed-off-by: Coly Li Link: https://lore.kernel.org/r/20220919161647.81238-4-colyli@suse.de Signed-off-by: Jens Axboe

bcache: Avoid comma separated statements

2021-02-10T15:06:00Z

Use semicolons and braces. Signed-off-by: Joe Perches Signed-off-by: Coly Li Signed-off-by: Jens Axboe

bcache: allocate meta data pages as compound pages

2020-07-25T13:38:19Z

There are some meta data of bcache are allocated by multiple pages, and they are used as bio bv_page for I/Os to the cache device. for example cache_set->uuids, cache->disk_buckets, journal_write->data, bset_tree->data. For such meta data memory, all the allocated pages should be treated as a single memory block. Then the memory management and underlying I/O code can treat them more clearly. This patch adds __GFP_COMP flag to all the location allocating >0 order pages for the above mentioned meta data. Then their pages are treated as compound pages now. Signed-off-by: Coly Li Cc: stable@vger.kernel.org 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: add code comments for state->pool in __btree_sort()

2020-01-23T18:40:02Z

To explain the pages allocated from mempool state->pool can be swapped in __btree_sort(), because state->pool is a page pool, which allocates pages by alloc_pages() indeed. Signed-off-by: Coly Li Signed-off-by: Jens Axboe