Re: [PATCH 0/4] blk-mq: support to use hw tag for scheduling

Ming Lei
2017-05-03T16:52:06Z

On Wed, May 03, 2017 at 11:38:09PM +0800, Ming Lei wrote:
> On Wed, May 03, 2017 at 09:08:34AM -0600, Jens Axboe wrote:
> > On 05/03/2017 09:03 AM, Ming Lei wrote:
> > > On Wed, May 03, 2017 at 08:10:58AM -0600, Jens Axboe wrote:
> > >> On 05/03/2017 08:08 AM, Jens Axboe wrote:
> > >>> On 05/02/2017 10:03 PM, Ming Lei wrote:
> > >>>> On Fri, Apr 28, 2017 at 02:29:18PM -0600, Jens Axboe wrote:
> > >>>>> On 04/28/2017 09:15 AM, Ming Lei wrote:
> > >>>>>> Hi,
> > >>>>>>
> > >>>>>> This patchset introduces flag of BLK_MQ_F_SCHED_USE_HW_TAG and
> > >>>>>> allows to use hardware tag directly for IO scheduling if the queue's
> > >>>>>> depth is big enough. In this way, we can avoid to allocate extra tags
> > >>>>>> and request pool for IO schedule, and the schedule tag allocation/release
> > >>>>>> can be saved in I/O submit path.
> > >>>>>
> > >>>>> Ming, I like this approach, it's pretty clean. It'd be nice to have a
> > >>>>> bit of performance data to back up that it's useful to add this code,
> > >>>>> though. Have you run anything on eg kyber on nvme that shows a
> > >>>>> reduction in overhead when getting rid of separate scheduler tags?
> > >>>>
> > >>>> I can observe small improvement in the following tests:
> > >>>>
> > >>>> 1) fio script
> > >>>> # io scheduler: kyber
> > >>>>
> > >>>> RWS="randread read randwrite write"
> > >>>> for RW in $RWS; do
> > >>>> echo "Running test $RW"
> > >>>> sudo echo 3 > /proc/sys/vm/drop_caches
> > >>>> sudo fio --direct=1 --size=128G --bsrange=4k-4k --runtime=20 --numjobs=1 --ioengine=libaio --iodepth=10240 --group_reporting=1 --filename=$DISK --name=$DISK-test-$RW --rw=$RW --output-format=json
> > >>>> done
> > >>>>
> > >>>> 2) results
> > >>>>
> > >>>> ---------------------------------------------------------
> > >>>> |sched tag(iops/lat) | use hw tag to sched(iops/lat)
> > >>>> ----------------------------------------------------------
> > >>>> randread |188940/54107 | 193865/52734
> > >>>> ----------------------------------------------------------
> > >>>> read |192646/53069 | 199738/51188
> > >>>> ----------------------------------------------------------
> > >>>> randwrite |171048/59777 | 179038/57112
> > >>>> ----------------------------------------------------------
> > >>>> write |171886/59492 | 181029/56491
> > >>>> ----------------------------------------------------------
> > >>>>
> > >>>> I guess it may be a bit more obvious when running the test on one slow
> > >>>> NVMe device, and will try to find one and run the test again.
> > >>>
> > >>> Thanks for running that. As I said in my original reply, I think this
> > >>> is a good optimization, and the implementation is clean. I'm fine with
> > >>> the current limitations of when to enable it, and it's not like we
> > >>> can't extend this later, if we want.
> > >>>
> > >>> I do agree with Bart that patch 1+4 should be combined. I'll do that.
> > >>
> > >> Actually, can you do that when reposting? Looks like you needed to
> > >> do that anyway.
> > >
> > > Yeah, I will do that in V1.
> >
> > V2? :-)
> >
> > Sounds good. I just wanted to check the numbers here, with the series
> > applied on top of for-linus crashes when switching to kyber. A few hunks
>
> Yeah, I saw that too, it has been fixed in my local tree, :-)
>
> > threw fuzz, but it looked fine to me. But I bet I fat fingered
> > something. So it'd be great if you could respin against my for-linus
> > branch.
>
> Actually, that is exactly what I am doing, :-)

Looks v4.11 plus your for-linus often triggers the following hang during
boot, and it seems caused by the change in (blk-mq: unify hctx delayed_run_work
and run_work)

UG: scheduling while atomic: kworker/0:1H/704/0x00000002
Modules linked in:
Preemption disabled at:
[<ffffffffaf5607bb>] virtio_queue_rq+0xdb/0x350
CPU: 0 PID: 704 Comm: kworker/0:1H Not tainted 4.11.0-04508-ga1f35f46164b #132
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.9.3-1.fc25 04/01/2014
Workqueue: kblockd blk_mq_run_work_fn
Call Trace:
dump_stack+0x65/0x8f
? virtio_queue_rq+0xdb/0x350
__schedule_bug+0x76/0xc0
__schedule+0x610/0x820
? new_slab+0x2c9/0x590
schedule+0x40/0x90
schedule_timeout+0x273/0x320
? ___slab_alloc+0x3cb/0x4f0
wait_for_completion+0x97/0x100
? wait_for_completion+0x97/0x100
? wake_up_q+0x80/0x80
flush_work+0x104/0x1a0
? flush_workqueue_prep_pwqs+0x130/0x130
__cancel_work_timer+0xeb/0x160
? vp_notify+0x16/0x20
? virtqueue_add_sgs+0x23c/0x4a0
cancel_delayed_work_sync+0x13/0x20
blk_mq_stop_hw_queue+0x16/0x20
virtio_queue_rq+0x316/0x350
blk_mq_dispatch_rq_list+0x194/0x350
blk_mq_sched_dispatch_requests+0x118/0x170
? finish_task_switch+0x80/0x1e0
__blk_mq_run_hw_queue+0xa3/0xc0
blk_mq_run_work_fn+0x2c/0x30
process_one_work+0x1e0/0x400
worker_thread+0x48/0x3f0
kthread+0x109/0x140
? process_one_work+0x400/0x400
? kthread_create_on_node+0x40/0x40
ret_from_fork+0x2c/0x40

Thansk,
Ming