Re: [PATCH 3/3] bcache: Fix the tail IO latency regression due to the use of lib min_heap

[Kuan-Wei Chiu <visitorckw@gmail.com>]

On Fri, Jun 06, 2025 at 12:19:45AM -0700, Robert Pang wrote:
> In commit "lib/min_heap: introduce non-inline versions of min heap API functions"
> (92a8b22), bcache migrates to the generic lib min_heap for all heap operations.
> This causes sizeable the tail IO latency regression during the cache replacement.

Nit: According to the documentation, I'd prefer referencing the commit
like this:

92a8b224b833 ("lib/min_heap: introduce non-inline versions of min heap
API functions")
https://docs.kernel.org/process/submitting-patches.html#describe-your-changes

Also, if the regression is caused by the heapify method, shouldn't the
commit that introduced it be 866898efbb25 ("bcache: remove heap-related
macros and switch to generic min_heap") ?

> 
> This commit updates invalidate_buckets_lru() to use the alternative APIs that
> sift down elements using the top-down approach like bcache's own original heap
> implementation.
> 
> [1] https://lore.kernel.org/linux-bcache/wtfuhfntbi6yorxqtpcs4vg5w67mvyckp2a6jmxuzt2hvbw65t@gznwsae5653d/T/#me50a9ddd0386ce602b2f17415e02d33b8e29f533
> 
> Signed-off-by: Robert Pang <robertpang@google.com>
> ---
>  drivers/md/bcache/alloc.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/md/bcache/alloc.c b/drivers/md/bcache/alloc.c
> index 8998e61efa40..547d1cd0c7c2 100644
> --- a/drivers/md/bcache/alloc.c
> +++ b/drivers/md/bcache/alloc.c
> @@ -207,15 +207,15 @@ static void invalidate_buckets_lru(struct cache *ca)
>  		if (!bch_can_invalidate_bucket(ca, b))
>  			continue;
>  
> -		if (!min_heap_full(&ca->heap))
> -			min_heap_push(&ca->heap, &b, &bucket_max_cmp_callback, ca);
> -		else if (!new_bucket_max_cmp(&b, min_heap_peek(&ca->heap), ca)) {
> +		if (!min_heap_full_inline(&ca->heap))
> +			min_heap_push_inline(&ca->heap, &b, &bucket_max_cmp_callback, ca);

If the regression is caused by the heapify method rather than the
inline vs non-inline change, is it necessary to switch to the
non-inline version here?

Regards,
Kuan-Wei

> +		else if (!new_bucket_max_cmp(&b, min_heap_peek_inline(&ca->heap), ca)) {
>  			ca->heap.data[0] = b;
> -			min_heap_sift_down(&ca->heap, 0, &bucket_max_cmp_callback, ca);
> +			min_heap_sift_down_top_down_inline(&ca->heap, 0, &bucket_max_cmp_callback, ca);
>  		}
>  	}
>  
> -	min_heapify_all(&ca->heap, &bucket_min_cmp_callback, ca);
> +	min_heapify_all_top_down_inline(&ca->heap, &bucket_min_cmp_callback, ca);
>  
>  	while (!fifo_full(&ca->free_inc)) {
>  		if (!ca->heap.nr) {
> @@ -227,8 +227,8 @@ static void invalidate_buckets_lru(struct cache *ca)
>  			wake_up_gc(ca->set);
>  			return;
>  		}
> -		b = min_heap_peek(&ca->heap)[0];
> -		min_heap_pop(&ca->heap, &bucket_min_cmp_callback, ca);
> +		b = min_heap_peek_inline(&ca->heap)[0];
> +		min_heap_pop_top_down_inline(&ca->heap, &bucket_min_cmp_callback, ca);
>  
>  		bch_invalidate_one_bucket(ca, b);
>  	}
> -- 
> 2.50.0.rc1.591.g9c95f17f64-goog
>