Re: Large latency on blk_queue_enter

[Ming Lei <ming.lei@redhat.com>]

On Mon, May 08, 2017 at 01:54:58PM +0200, Javier Gonz�lez wrote:
> Hi,
> 
> I find an unusual added latency(~20-30ms) on blk_queue_enter when
> allocating a request directly from the NVMe driver through
> nvme_alloc_request. I could use some help confirming that this is a bug
> and not an expected side effect due to something else.
> 
> I can reproduce this latency consistently on LightNVM when mixing I/O
> from pblk and I/O sent through an ioctl using liblightnvm, but I don't
> see anything on the LightNVM side that could impact the request
> allocation.
> 
> When I have a 100% read workload sent from pblk, the max. latency is
> constant throughout several runs at ~80us (which is normal for the media
> we are using at bs=4k, qd=1). All pblk I/Os reach the nvme_nvm_submit_io
> function on lightnvm.c., which uses nvme_alloc_request. When we send a
> command from user space through an ioctl, then the max latency goes up
> to ~20-30ms. This happens independently from the actual command
> (IN/OUT). I tracked down the added latency down to the call
> percpu_ref_tryget_live in blk_queue_enter. Seems that the queue
> reference counter is not released as it should through blk_queue_exit in
> blk_mq_alloc_request. For reference, all ioctl I/Os reach the
> nvme_nvm_submit_user_cmd on lightnvm.c
> 
> Do you have any idea about why this might happen? I can dig more into
> it, but first I wanted to make sure that I am not missing any obvious
> assumption, which would explain the reference counter to be held for a
> longer time.

You need to check if the .q_usage_counter is working at atomic mode.
This counter is initialized as atomic mode, and finally switchs to
percpu mode via percpu_ref_switch_to_percpu() in blk_register_queue().

Thanks,
Ming