Re: [PATCH 0/3] scsi: scsi-mq: don't hold host_busy in IO path

Jens Axboe
2018-04-27T16:19:54Z

On 4/27/18 9:48 AM, Bart Van Assche wrote:
> On Fri, 2018-04-27 at 09:39 -0600, Jens Axboe wrote:
>> blk_mq_tagset_busy_iter(&shost->tag_set, scsi_host_check_in_flight,
>> &in_flight);
>> return in_flight.cnt + atomic_read(&shost->host_busy);
>>
>> The atomic read is basically free, once we get rid of the dirty of that
>> variable on each IO.
>
> Hello Jens,
>
> What makes you think that " + atomic_read(&shost->host_busy)" is necessary?
> I am not aware of any code outside the SCSI core that modifies the host_busy
> member.

It's a (scalable) hack to count those as well. Going forward they should be
converted to just using reserved tags, of course.

--
Jens Axboe