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

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

On Wed, Apr 14, 2021 at 04:12:22PM +0530, Kashyap Desai wrote:
> > Hi Ming,
> >
> > >
> > > It is reported inside RH that CPU utilization is increased ~20% when
> > > running simple FIO test inside VM which disk is built on image stored
> > > on XFS/megaraid_sas.
> > >
> > > When I try to investigate by reproducing the issue via scsi_debug, I
> > > found IO hang when running randread IO(8k, direct IO, libaio) on
> > > scsi_debug disk created by the following command:
> > >
> > > 	modprobe scsi_debug host_max_queue=128
> > submit_queues=$NR_CPUS
> > > virtual_gb=256
> > >
> >
> > So I can recreate this hang for using mq-deadline IO sched for scsi debug,
> > in
> > that fio does not exit. I'm using v5.12-rc7.
> 
> I can also recreate this issue using mq-deadline. Using <none>, there is no
> IO hang issue.
> Also if I run script to change scheduler periodically (none, mq-deadline),
> sysfs entry hangs.
> 
> Here is call trace-
> Call Trace:
> [ 1229.879862]  __schedule+0x29d/0x7a0
> [ 1229.879871]  schedule+0x3c/0xa0
> [ 1229.879875]  blk_mq_freeze_queue_wait+0x62/0x90
> [ 1229.879880]  ? finish_wait+0x80/0x80
> [ 1229.879884]  elevator_switch+0x12/0x40
> [ 1229.879888]  elv_iosched_store+0x79/0x120
> [ 1229.879892]  ? kernfs_fop_write_iter+0xc7/0x1b0
> [ 1229.879897]  queue_attr_store+0x42/0x70
> [ 1229.879901]  kernfs_fop_write_iter+0x11f/0x1b0
> [ 1229.879905]  new_sync_write+0x11f/0x1b0
> [ 1229.879912]  vfs_write+0x184/0x250
> [ 1229.879915]  ksys_write+0x59/0xd0
> [ 1229.879917]  do_syscall_64+0x33/0x40
> [ 1229.879922]  entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> 
> I tried both - 5.12.0-rc1 and 5.11.0-rc2+ and there is a same behavior.
> Let me also check  megaraid_sas and see if anything generic or this is a
> special case of scsi_debug.

As I mentioned, it could be one generic issue wrt. SCHED_RESTART.
shared tags might have to restart all hctx since all share same tags.


Thanks,
Ming