Re: [bug report] shared tags causes IO hang and performance drop

[John Garry <john.garry@huawei.com>]

On 26/04/2021 15:48, Ming Lei wrote:
>>       --0.56%--sbitmap_get
>>
>> I don't see this for hostwide tags - this may be because we have multiple
>> hctx, and the IO sched tags are per hctx, so less chance of exhaustion. But
>> this is not from hostwide tags specifically, but for multiple HW queues in
>> general. As I understood, sched tags were meant to be per request queue,
>> right? I am reading this correctly?
> sched tags is still per-hctx.
> 
> I just found that you didn't change sched tags into per-request-queue
> shared tags. 
> Then for hostwide tags, each hctx still has its own
> standalone sched tags and request pool, that is one big difference with
> non hostwide tags. 

For both hostwide and non-hostwide tags, we have standalone sched tags 
and request pool per hctx when q->nr_hw_queues > 1.

> That is why you observe that scheduler tag exhaustion
> is easy to trigger in case of non-hostwide tags.
> 
> I'd suggest to add one per-request-queue sched tags, and make all hctxs
> sharing it, just like what you did for driver tag.
> 

That sounds reasonable.

But I don't see how this is related to hostwide tags specifically, but 
rather just having q->nr_hw_queues > 1, which NVMe PCI and some other 
SCSI MQ HBAs have (without using hostwide tags).

Thanks,
John