Re: [PATCH V6 3/5] blk-mq: ensure hctx to be ran on mapped cpu when issue directly

[Jens Axboe]

On 11/13/18 2:56 AM, Jianchao Wang wrote:
> When issue request directly and the task is migrated out of the
> original cpu where it allocates request, hctx could be ran on
> the cpu where it is not mapped.
> To fix this,
>  - insert the request forcibly if BLK_MQ_F_BLOCKING is set.
>  - check whether the current is mapped to the hctx, if not, insert
>    forcibly.
>  - invoke __blk_mq_issue_directly under preemption disabled.

I'm not too crazy about this one, adding a get/put_cpu() in the hot
path, and a cpumask test. The fact is that most/no drivers care
about strict placement. We always try to do so, if convenient,
since it's faster, but this seems to be doing the opposite.

I'd be more inclined to have a driver flag if it needs guaranteed
placement, using one an ops BLK_MQ_F_STRICT_CPU flag or similar.

What do you think?

-- 
Jens Axboe