Re: [RFC PATCH] block: set bdi congested when no tag available

[Vlastimil Babka <vbabka@xxxxxxx>]

On 3/9/22 08:40, Zhaoyang Huang wrote:
> From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> 
> In previous version, block layer will set bdi to be congested when
> get_request fail, which may throttle direct_reclaim. Move them back
> under current blk-mq design.

Hm I thought Mel removed the direct reclaim throttling based on bdi
congestion in his series [1]. Maybe block layers has other uses (I have no
idea), for tracking congestion, that should be mentioned instead?

[1]
https://lore.kernel.org/all/20211022144651.19914-1-mgorman@xxxxxxxxxxxxxxxxxxx/

> 
> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> ---
>  block/blk-mq-tag.c      | 18 +++++++++++++++++-
>  include/linux/sbitmap.h |  1 +
>  lib/sbitmap.c           | 17 +++++++++++++++++
>  3 files changed, 35 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index 995336a..bd1e520 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -11,6 +11,7 @@
> 
>  #include <linux/blk-mq.h>
>  #include <linux/delay.h>
> +#include <linux/backing-dev.h>
>  #include "blk.h"
>  #include "blk-mq.h"
>  #include "blk-mq-sched.h"
> @@ -126,8 +127,11 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
>   if (tag != BLK_MQ_NO_TAG)
>   goto found_tag;
> 
> - if (data->flags & BLK_MQ_REQ_NOWAIT)
> + if (data->flags & BLK_MQ_REQ_NOWAIT) {
> + set_bdi_congested(data->q->disk->bdi,BLK_RW_SYNC);
> + set_bdi_congested(data->q->disk->bdi,BLK_RW_ASYNC);
>   return BLK_MQ_NO_TAG;
> + }
> 
>   ws = bt_wait_ptr(bt, data->hctx);
>   do {
> @@ -193,9 +197,21 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
>   return tag + tag_offset;
>  }
> 
> +static bool blk_mq_has_free_tags(struct blk_mq_tags *tags)
> +{
> + if (!tags)
> + return true;
> +
> + return sbitmap_any_bit_clear(&tags->bitmap_tags.sb);
> +}
> +
>  void blk_mq_put_tag(struct blk_mq_tags *tags, struct blk_mq_ctx *ctx,
>       unsigned int tag)
>  {
> + if (!blk_mq_has_free_tags(tags)) {
> + clear_bdi_congested(ctx->queue->disk->bdi, BLK_RW_SYNC);
> + clear_bdi_congested(ctx->queue->disk->bdi, BLK_RW_ASYNC);
> + }
>   if (!blk_mq_tag_is_reserved(tags, tag)) {
>   const int real_tag = tag - tags->nr_reserved_tags;
> 
> diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
> index 4a6ff27..30a5553 100644
> --- a/include/linux/sbitmap.h
> +++ b/include/linux/sbitmap.h
> @@ -210,6 +210,7 @@ static inline void sbitmap_free(struct sbitmap *sb)
>   * Return: true if any bit in the bitmap is set, false otherwise.
>   */
>  bool sbitmap_any_bit_set(const struct sbitmap *sb);
> +bool sbitmap_any_bit_clear(const struct sbitmap *sb);
> 
>  #define SB_NR_TO_INDEX(sb, bitnr) ((bitnr) >> (sb)->shift)
>  #define SB_NR_TO_BIT(sb, bitnr) ((bitnr) & ((1U << (sb)->shift) - 1U))
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 2709ab8..baa30d8 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -309,6 +309,23 @@ bool sbitmap_any_bit_set(const struct sbitmap *sb)
>  }
>  EXPORT_SYMBOL_GPL(sbitmap_any_bit_set);
> 
> +bool sbitmap_any_bit_clear(const struct sbitmap *sb)
> +{
> + unsigned int i;
> +
> + for (i = 0; i < sb->map_nr; i++) {
> + const struct sbitmap_word *word = &sb->map[i];
> + unsigned long mask = word->word & ~word->cleared;
> + unsigned long ret;
> +
> + ret = find_first_zero_bit(&mask, word->depth);
> + if (ret < word->depth)
> + return true;
> + }
> + return false;
> +}
> +EXPORT_SYMBOL_GPL(sbitmap_any_bit_clear);
> +
>  static unsigned int __sbitmap_weight(const struct sbitmap *sb, bool set)
>  {
>   unsigned int i, weight = 0;