linux-kernel - Re: [PATCH] block: plug attempts to batch allocate tags multiple times

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Message-Id: <20250912030601.236426-1-xue01.he@samsung.com>
Date: Fri, 12 Sep 2025 03:06:01 +0000
From: Xue He <xue01.he@...sung.com>
To: yukuai1@...weicloud.com
Cc: axboe@...nel.dk, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, xue01.he@...sung.com, yukuai3@...wei.com
Subject: Re: [PATCH] block: plug attempts to batch allocate tags multiple
 times

On 2025/09/03 18:35 PM, Yu Kuai wrote:
>On 2025/09/03 16:41 PM, Xue He wrote:
>> On 2025/09/02 08:47 AM, Yu Kuai wrote:
>>> On 2025/09/01 16:22, Xue He wrote:
>> ......
>
>
>Yes, so this function will likely to obtain less tags than nr_tags,the
>mask is always start from first zero bit with nr_tags bit, and
>sbitmap_deferred_clear() is called uncondionally, it's likely there are
>non-zero bits within this range.
>
>Just wonder, do you consider fixing this directly in
>__blk_mq_alloc_requests_batch()?
>
>  - call sbitmap_deferred_clear() and retry on allocation failure, so
>that the whole word can be used even if previous allocated request are
>done, especially for nvme with huge tag depths;
>  - retry blk_mq_get_tags() until data->nr_tags is zero;

Hi, Yu Kuai, I'm not entirely sure if I understand correctly, but during
each tag allocation, sbitmap_deferred_clear() is typically called first, 
as seen in the __sbitmap_queue_get_batch() function.

        for (i = 0; i < sb->map_nr; i++) {
                struct sbitmap_word *map = &sb->map[index];
                unsigned long get_mask;
                unsigned int map_depth = __map_depth(sb, index);
                unsigned long val;

                sbitmap_deferred_clear(map, 0, 0, 0);
------------------------------------------------------------------------
so I try to recall blk_mq_get_tags() until data->nr_tags is zero, like:

-       int i, nr = 0;

-       tag_mask = blk_mq_get_tags(data, data->nr_tags, &tag_offset);
-       if (unlikely(!tag_mask))
-               return NULL;
-
-       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;
+       do {
+               int i, nr = 0;
+               tag_mask = blk_mq_get_tags(data, data->nr_tags, &tag_offset);
+               if (unlikely(!tag_mask))
+                       return NULL;
+               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;
+       } while (data->nr_tags);

I added a loop structure, it also achieve a good results like before,
but I have a question: although the loop will retry tag allocation
when the required number of tags is not met, there is a risk of an
infinite loop, right? However, I couldn't think of a safer condition
to terminate the loop. Do you have any suggestions?

Thanks,
Xue