blk-mq request allocation stalls [was: Re: [PATCH v3 0/8] dm: add request-based blk-mq support]

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Jan 06 2015 at  4:31am -0500,
Bart Van Assche <bvanassche@acm.org> wrote:

> On 01/05/15 22:35, Mike Snitzer wrote:
> > On Fri, Jan 02 2015 at 12:53pm -0500,
> > Bart Van Assche <bvanassche@acm.org> wrote:
> >> Thanks, my tests confirm that this patch indeed fixes the issue I had
> >> reported. Unfortunately this doesn't mean that the blk-mq multipath code
> >> is already working perfectly. Most of the time I/O requests are
> >> processed within the expected time but sometimes I/O processing takes
> >> much more time than what I expected:
> >>
> >> # /usr/bin/time -f %e mkfs.xfs -f /dev/dm-0 >/dev/null
> >> 0.02
> >> # /usr/bin/time -f %e mkfs.xfs -f /dev/dm-0 >/dev/null
> >> 0.02
> >> # /usr/bin/time -f %e mkfs.xfs -f /dev/dm-0 >/dev/null
> >> 8.68
> >>
> >> However, if I run the same command on the underlying device it always
> >> completes within the expected time.
> > 
> > I don't have very large blk-mq devices, but I can work on that.
> > How large is the blk-mq device in question?
> > 
> > Also, how much memory does the system have?  Is memory fragmented at
> > all?  With this change the requests are cloned using memory allocated
> > from block core's blk_get_request (rather than a dedicated mempool in DM
> > core).
> > 
> > Any chance you could use 'perf record' to try to analyze where the
> > kernel is spending its time?
> 
> Hello Mike,
> 
> The device used in this test was a tmpfs file with a size of 16 MB. That
> file had been created as follows: dd if=/dev/zero of=/dev/vdisk bs=1M
> count=16. The initiator and target systems did have enough memory to keep
> this tmpfs file in RAM all the time (32 GB and 4 GB respectively).
> 
> For the runs that took much longer than expected the CPU load was low.
> This probably means that the system was waiting for one or another I/O
> timer to expire. The output triggered by "echo w > /proc/sysrq-trigger"
> during a run that took longer than expected was as follows:
> 
> SysRq : Show Blocked State
>   task                        PC stack   pid father
> kdmwork-253:0   D ffff8807c1fd3b78     0 10396      2 0x00000000
>  ffff8807c1fd3b78 ffff88083b6b6cc0 0000000000012ec0 ffff8807c1fd3fd8
>  0000000000012ec0 ffff880824225aa0 ffff88083b6b6cc0 ffff88081b0cb2c0
>  ffff88085fc537c8 ffff8807c1fd3c98 ffff8807f7a99d70 ffffe8ffffc43bc0
> Call Trace:
>  [<ffffffff814d5230>] io_schedule+0xa0/0x130
>  [<ffffffff8125a3f7>] bt_get+0x117/0x1b0
>  [<ffffffff81256580>] ? blk_mq_queue_enter+0x30/0x2a0
>  [<ffffffff81094cf0>] ? prepare_to_wait_event+0x110/0x110
>  [<ffffffff8125a76f>] blk_mq_get_tag+0x9f/0xd0
>  [<ffffffff8125591b>] __blk_mq_alloc_request+0x1b/0x210
>  [<ffffffff812571c9>] blk_mq_alloc_request+0x139/0x150
>  [<ffffffff8124c16e>] blk_get_request+0x2e/0xe0
>  [<ffffffff8109a60d>] ? trace_hardirqs_on+0xd/0x10
>  [<ffffffffa07f7d0f>] __multipath_map.isra.15+0x1cf/0x210 [dm_multipath]
>  [<ffffffffa07f7d6a>] multipath_clone_and_map+0x1a/0x20 [dm_multipath]
>  [<ffffffffa039dbb5>] map_tio_request+0x1d5/0x3a0 [dm_mod]
>  [<ffffffff8109a53d>] ? trace_hardirqs_on_caller+0xfd/0x1c0
>  [<ffffffff81075cbe>] kthread_worker_fn+0x7e/0x1b0
>  [<ffffffff81075c40>] ? __init_kthread_worker+0x60/0x60
>  [<ffffffff81075bc8>] kthread+0xf8/0x110
>  [<ffffffff81075ad0>] ? kthread_create_on_node+0x210/0x210
>  [<ffffffff814dacac>] ret_from_fork+0x7c/0xb0
>  [<ffffffff81075ad0>] ? kthread_create_on_node+0x210/0x210

Jens,

This stack trace confirms my suspicion that switching DM-multipath over
to allocating clone requests via blk_get_request (rather than using a
dedicated mempool in DM core) is the cause of the slowdown that Bart has
experienced.

Given blk_mq_get_tag() looks to be the culprit is there anything we can
do to speed up blk-mq request allocation?  I'm currently using
GFP_KERNEL when calling blk_get_request().

Mike