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

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Date:   Thu, 27 Oct 2022 10:16:43 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     John Garry <john.garry@...wei.com>, axboe@...nel.dk,
        jejb@...ux.ibm.com, martin.petersen@...cle.com,
        jinpu.wang@...ud.ionos.com, hare@...e.de, bvanassche@....org,
        hch@....de, ming.lei@...hat.com, niklas.cassel@....com
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-ide@...r.kernel.org, linux-scsi@...r.kernel.org,
        linuxarm@...wei.com
Subject: Re: [PATCH RFC v3 01/22] blk-mq: Don't get budget for reserved
 requests

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@...wei.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