Re: [PATCH V2] blk-mq: avoid to starve tag allocation after allocation process migrates

[Jens Axboe <axboe@kernel.dk>]

On 5/22/18 6:22 PM, Ming Lei wrote:
> On Tue, May 22, 2018 at 02:20:37PM -0600, Jens Axboe wrote:
>> On 5/19/18 1:44 AM, Ming Lei wrote:
>>> When the allocation process is scheduled back and the mapped hw queue is
>>> changed, do one extra wake up on orignal queue for compensating wake up
>>> miss, so other allocations on the orignal queue won't be starved.
>>>
>>> This patch fixes one request allocation hang issue, which can be
>>> triggered easily in case of very low nr_request.
>>
>> Can you explain what the actual bug is? The commit message doesn't
>> really say, it just explains what the code change does. What wake up
>> miss?
> 
> For example:
> 
> 1) one request queue has 2 queues, and queue depth is low, so wake_batch
> is set 1 or very small.

We have too many 'queues' :)

> 2) There are 2 allocations which are blocked on hw queue 0, now the last
> request mapped to hw queue 0 is completed, then sbq_wake_up() wakes only
> one of 2 blockers.
> 
> 3) the waken up allocation process is migrated to another CPU, and its
> hw queue becomes mapped to hw queue 1, and this allocation is switched
> to hw queue 1
> 
> 4) so there isn't any chance for another blocked allocation to move one,
> since all request in hw queue 0 has been completed. That is why I call
> it wake up miss.

OK, that makes sense to me. With that, let me review your patch in
a different email.

-- 
Jens Axboe