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

commit 48fd7ebe upstream.

This reverts commit 866898ef.

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 92a8b224 ("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: 866898ef ("bcache: remove heap-related macros and switch to generic min_heap")
Fixes: 92a8b224 ("lib/min_heap: introduce non-inline versions of min heap API functions")
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Reported-by: Robert Pang <robertpang@google.com>
Closes: https://lore.kernel.org/linux-bcache/CAJhEC06F_AtrPgw2-7CvCqZgeStgCtitbD-ryuPpXQA-JG5XXw@mail.gmail.com


Acked-by: Coly Li <colyli@kernel.org>
Cc: Ching-Chun (Jim) Huang <jserv@ccns.ncku.edu.tw>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>