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Message-ID: <979713cc-b2f6-00fa-7021-17d0e74a6913@huaweicloud.com>
Date: Tue, 14 Oct 2025 14:28:55 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Xue He <xue01.he@...sung.com>, axboe@...nel.dk,
akpm@...ux-foundation.org, yukuai1@...weicloud.com
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
"yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v4] block: plug attempts to batch allocate tags multiple
times
Hi,
在 2025/10/10 14:35, Xue He 写道:
> This patch aims to enable batch allocation of sufficient tags after
> batch IO submission with plug mechanism, thereby avoiding the need for
> frequent individual requests when the initial allocation is
> insufficient.
>
> ------------------------------------------------------------
> Perf:
> base code: __blk_mq_alloc_requests() 1.31%
> patch: __blk_mq_alloc_requests() 0.71%
> ------------------------------------------------------------
>
> ---
> changes since v1:
> - Modify multiple batch registrations into a single loop to achieve
> the batch quantity
>
> changes since v2:
> - Modify the call location of remainder handling
> - Refactoring sbitmap cleanup time
>
> changes since v3:
> - Add handle operation in loop
> - Add helper sbitmap_find_bits_in_word
>
> Signed-off-by: hexue <xue01.he@...sung.com>
> ---
> block/blk-mq.c | 39 ++++++++++++++++++---------------
> lib/sbitmap.c | 58 ++++++++++++++++++++++++++++++++++----------------
> 2 files changed, 62 insertions(+), 35 deletions(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index ba3a4b77f578..695ccc72e69f 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -458,26 +458,31 @@ __blk_mq_alloc_requests_batch(struct blk_mq_alloc_data *data)
> unsigned long tag_mask;
> int i, nr = 0;
>
> - tag_mask = blk_mq_get_tags(data, data->nr_tags, &tag_offset);
> - if (unlikely(!tag_mask))
> - return NULL;
> + do {
> + tag_mask = blk_mq_get_tags(data, data->nr_tags, &tag_offset);
> + if (unlikely(!tag_mask)) {
> + if (nr == 0)
> + return NULL;
> + break;
> + }
> + tags = blk_mq_tags_from_data(data);
> + for (i = 0; tag_mask; i++) {
> + if (!(tag_mask & (1UL << i)))
> + continue;
> + tag = tag_offset + i;
> + prefetch(tags->static_rqs[tag]);
> + tag_mask &= ~(1UL << i);
> + rq = blk_mq_rq_ctx_init(data, tags, tag);
> + rq_list_add_head(data->cached_rqs, rq);
> + data->nr_tags--;
> + nr++;
> + }
> + if (!(data->rq_flags & RQF_SCHED_TAGS))
> + blk_mq_add_active_requests(data->hctx, nr);
> + } while (data->nr_tags);
>
> - tags = blk_mq_tags_from_data(data);
> - for (i = 0; tag_mask; i++) {
> - if (!(tag_mask & (1UL << i)))
> - continue;
> - tag = tag_offset + i;
> - prefetch(tags->static_rqs[tag]);
> - tag_mask &= ~(1UL << i);
> - rq = blk_mq_rq_ctx_init(data, tags, tag);
> - rq_list_add_head(data->cached_rqs, rq);
> - nr++;
> - }
> - if (!(data->rq_flags & RQF_SCHED_TAGS))
> - blk_mq_add_active_requests(data->hctx, nr);
> /* caller already holds a reference, add for remainder */
> percpu_ref_get_many(&data->q->q_usage_counter, nr - 1);
> - data->nr_tags -= nr;
>
> return rq_list_pop(data->cached_rqs);
> }
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 4d188d05db15..c0a8da1beec9 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -208,6 +208,32 @@ static int sbitmap_find_bit_in_word(struct sbitmap_word *map,
> return nr;
> }
>
> +static unsigned long sbitmap_find_bits_in_word(struct sbitmap_word *map,
> + unsigned int depth, unsigned int alloc_hint, bool wrap,
> + unsigned long *val, int nr_tags, unsigned int map_depth)
The parameters depth, alloc_hint, wrap are not necessary, the only
caller always pass in 0.
> +{
> + unsigned long local_val, nr;
> +
> + while (1) {
> + local_val = READ_ONCE(map->word);
> + if (local_val == (1UL << (map_depth - 1)) - 1) {
> + if (!sbitmap_deferred_clear(map, depth, alloc_hint, wrap))
> + return -1UL;
> + continue;
> + }
> +
> + nr = find_first_zero_bit(&local_val, map_depth);
> + if (nr + nr_tags <= map_depth)
> + break;
With the respect we can allocate multiple times, and it's not necessary
to allocate nr_tags at once, perhaps it'll make sense to skip this
checking.
> +
> + if (!sbitmap_deferred_clear(map, depth, alloc_hint, wrap))
> + return -1UL;
> + };
> +
> + *val = local_val;
> + return nr;
> +}
> +
> static unsigned int __map_depth_with_shallow(const struct sbitmap *sb,
> int index,
> unsigned int shallow_depth)
> @@ -534,26 +560,22 @@ unsigned long __sbitmap_queue_get_batch(struct sbitmap_queue *sbq, int nr_tags,
> unsigned int map_depth = __map_depth(sb, index);
> unsigned long val;
>
> - sbitmap_deferred_clear(map, 0, 0, 0);
> - val = READ_ONCE(map->word);
> - if (val == (1UL << (map_depth - 1)) - 1)
> + nr = sbitmap_find_bits_in_word(map, 0, 0, 0, &val, nr_tags, map_depth);
> + if (nr == -1UL)
> goto next;
>
> - nr = find_first_zero_bit(&val, map_depth);
> - if (nr + nr_tags <= map_depth) {
> - atomic_long_t *ptr = (atomic_long_t *) &map->word;
> -
> - get_mask = ((1UL << nr_tags) - 1) << nr;
> - while (!atomic_long_try_cmpxchg(ptr, &val,
> - get_mask | val))
> - ;
> - get_mask = (get_mask & ~val) >> nr;
> - if (get_mask) {
> - *offset = nr + (index << sb->shift);
> - update_alloc_hint_after_get(sb, depth, hint,
> - *offset + nr_tags - 1);
> - return get_mask;
> - }
> + atomic_long_t *ptr = (atomic_long_t *) &map->word;
> +
> + get_mask = ((1UL << nr_tags) - 1) << nr;
> + while (!atomic_long_try_cmpxchg(ptr, &val,
> + get_mask | val))
> + ;
> + get_mask = (get_mask & ~val) >> nr;
> + if (get_mask) {
> + *offset = nr + (index << sb->shift);
> + update_alloc_hint_after_get(sb, depth, hint,
> + *offset + nr_tags - 1);
> + return get_mask;
I feel it's better to fold in above into the helper and return get_mask
directly.
BTW, the sbitmap and blk-mq changes are not quite related, and you can
split them into seperate patches.
Thanks,
Kuai
> }
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