Re: [PATCH RFC v3 01/22] blk-mq: Don't get budget for reserved requests

Damien Le Moal
2022-10-27T01:16:52Z

On 10/25/22 19:17, John Garry wrote:
> It should be possible to send reserved requests even when there is no
> budget, so don't request a budget in that case.
>
> This comes into play when we need to allocate a reserved request from the
> target device request queue for error handling for that same device.
>
> Signed-off-by: John Garry <john.garry@huawei.com>
> ---
> block/blk-mq.c | 4 +++-
> drivers/scsi/scsi_lib.c | 3 ++-
> 2 files changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 260adeb2e455..d8baabb32ea4 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1955,11 +1955,13 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list,
> errors = queued = 0;
> do {
> struct blk_mq_queue_data bd;
> + bool need_budget;
>
> rq = list_first_entry(list, struct request, queuelist);
>
> WARN_ON_ONCE(hctx != rq->mq_hctx);
> - prep = blk_mq_prep_dispatch_rq(rq, !nr_budgets);
> + need_budget = !nr_budgets && !blk_mq_is_reserved_rq(rq);
> + prep = blk_mq_prep_dispatch_rq(rq, need_budget);
> if (prep != PREP_DISPATCH_OK)
> break;

Below this code, there is:

if (nr_budgets)
nr_budgets--;

Don't you need to change that to:

if (need_budget && nr_budgets)
nr_budgets--;

? Otherwise, the accounting will be off.

>
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index fa96d3cfdfa3..39d4fd124375 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -298,7 +298,8 @@ void scsi_device_unbusy(struct scsi_device *sdev, struct scsi_cmnd *cmd)
> if (starget->can_queue > 0)
> atomic_dec(&starget->target_busy);
>
> - sbitmap_put(&sdev->budget_map, cmd->budget_token);
> + if (!blk_mq_is_reserved_rq(scsi_cmd_to_rq(cmd)))
> + sbitmap_put(&sdev->budget_map, cmd->budget_token);
> cmd->budget_token = -1;
> }
>

--
Damien Le Moal
Western Digital Research