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 10:09:35 +0100
From:   John Garry <john.garry@...wei.com>
To:     Damien Le Moal <damien.lemoal@...nsource.wdc.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 27/10/2022 02:16, Damien Le Moal wrote:
>> 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.
> 

Ah, yes, I think that you are right. I actually need to check nr_budgets 
usage further as nr_budgets initial value would be dependent on any 
reserved request requiring a budget (which we don't get).

Thanks,
John