On Fri 10-02-23 04:11:11, Kemeng Shi wrote:
> Commit d263ed9926823 ("blk-mq: count the hctx as active before allocating
> tag") active hctx before blk_mq_get_tag to avoid petential starvation.
> However, the hctx to alloc tag may change in blk_mq_get_tag if
> BLK_MQ_REQ_NOWAIT is not set, then there are two problems:
> 1. The hctx without real allocation is marked active.
> 2. The starvation problem mentioned in Commit d263ed9926823 ("blk-mq:
> count the hctx as active before allocating tag") still exists on the
> changed hctx as it maybe not marked active before tag allocation.
>
> For problem 1, the hctx which is marked active probably gets IO
> soon or will be marked inactive after lazy detection of tag idle.
> Mark changed hctx active to fix problem 2.
>
> Fixes: d263ed992682 ("blk-mq: count the hctx as active before allocating tag")
> Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
Yeah, makes sense to me. Feel free to add:
Reviewed-by: Jan Kara <jack@xxxxxxx>
Honza
> ---
> block/blk-mq-tag.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index 1d3135acfc98..e566fd96dc26 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -191,6 +191,9 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
> data->ctx = blk_mq_get_ctx(data->q);
> data->hctx = blk_mq_map_queue(data->q, data->cmd_flags,
> data->ctx);
> + if (!(data->rq_flags & RQF_ELV))
> + blk_mq_tag_busy(data->hctx);
> +
> tags = blk_mq_tags_from_data(data);
> if (data->flags & BLK_MQ_REQ_RESERVED)
> bt = &tags->breserved_tags;
> --
> 2.30.0
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
