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

[jianchao.wang]

On 11/14/18 11:02 AM, Ming Lei wrote:
> On Wed, Nov 14, 2018 at 10:15 AM jianchao.wang
> <jianchao.w.wang@xxxxxxxxxx> wrote:
>>
>> Hi Jens
>>
>> Thanks for your kindly response.
>>
>> On 11/13/18 9:44 PM, Jens Axboe wrote:
>>> 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?
>>>
>>
>> I'd inclined blk-mq should comply with a unified rule, no matter the
>> issuing directly path or inserting one. Then blk-mq would have a simpler
>> model. And also this guarantee could be a little good for drivers,
>> especially the case where cpu and hw queue mapping is 1:1.
> 
> I guess it is quite hard to respect this rule 100%, such as in case of
> CPU hotplug.
> 
Yes, it is indeed the case.

Looks like this patch is contentious.
I will drop this one and post later as a standalone one if necessary.

Thanks
Jianchao