On 7/4/19 5:21 AM, Filipp Mikoian wrote:
> Hi dear io_uring developers,
>
> Recently I started playing with io_uring, and the main difference I expected
> to see with old AIO(io_submit(), etc.) was submission syscall(io_uring_enter())
> not blocking in case submission might take long time, e.g. if waiting for a slot
> in block device request queue is required. AFAIU, 'workers' machinery is used
> solely to be able to submit requests in async context, thus not forcing calling
> thread to block for a significant time. At worst EAGAIN is expected.
>
> However, when I installed fresh 5.2.0-rc7 kernel on the machine with HDD with
> 64-requests-deep queue, I noticed significant increase in time spent in
> io_uring_enter() once request queue became full. Below you can find output
> of the program that submits random(in 1GB range) 4K read requests in batches
> of 32. Though O_DIRECT is used, the same phenomenon is observed when using
> page cache. Source code can be found here:
> https://github.com/Phikimon/io_uring_question
>
> While analyzing stack dump, I found out that IOCB_NOWAIT flag being set
> does not prevent generic_file_read_iter() from calling blkdev_direct_IO(),
> so thread gets stuck for hundreds of milliseconds. However, I am not a
> Linux kernel expert, so I can not be sure this is actually related to the
> mentioned issue.
>
> Is it actually expected that io_uring would sleep in case there is no slot
> in block device's request queue, or is this a bug of current implementation?
>
> root@localhost:~/io_uring# uname -msr
> Linux 5.2.0-rc7 x86_64
> root@localhost:~/io_uring# hdparm -I /dev/sda | grep Model
> Model Number: Hitachi HTS541075A9E680
> root@localhost:~/io_uring# cat /sys/block/sda/queue/nr_requests
> 64
> root@localhost:~/io_uring# ./io_uring_read_blkdev /dev/sda8
> submitted_already = 0, submitted_now = 32, submit_time = 246 us
> submitted_already = 32, submitted_now = 32, submit_time = 130 us
> submitted_already = 64, submitted_now = 32, submit_time = 189548 us
> submitted_already = 96, submitted_now = 32, submit_time = 121542 us
> submitted_already = 128, submitted_now = 32, submit_time = 128314 us
> submitted_already = 160, submitted_now = 32, submit_time = 136345 us
> submitted_already = 192, submitted_now = 32, submit_time = 162320 us
> root@localhost:~/io_uring# cat pstack_output # This is where process slept
> [<0>] io_schedule+0x16/0x40
> [<0>] blk_mq_get_tag+0x166/0x280
> [<0>] blk_mq_get_request+0xde/0x380
> [<0>] blk_mq_make_request+0x11e/0x5b0
> [<0>] generic_make_request+0x191/0x3c0
> [<0>] submit_bio+0x75/0x140
> [<0>] blkdev_direct_IO+0x3f8/0x4a0
> [<0>] generic_file_read_iter+0xbf/0xdc0
> [<0>] blkdev_read_iter+0x37/0x40
> [<0>] io_read+0xf6/0x180
> [<0>] __io_submit_sqe+0x1cd/0x6a0
> [<0>] io_submit_sqe+0xea/0x4b0
> [<0>] io_ring_submit+0x86/0x120
> [<0>] __x64_sys_io_uring_enter+0x241/0x2d0
> [<0>] do_syscall_64+0x60/0x1a0
> [<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
> [<0>] 0xffffffffffffffff
Sorry for originally missing this one! For this particular issue, it
looks like you are right, we don't honor NOWAIT for request allocation.
That's a bug, pondering how best to fix this. Can you try the attached
patch and see if it fixes it for you?
> 1. Inaccurate handling of errors in liburing/__io_uring_submit().
> Because liburing currently does not care about queue head that kernel
> sets, it cannot know how many entries have been actually consumed. In
> case e.g. io_uring_enter() returns EAGAIN, and consumes none of the
> sqes, sq->sqe_head still advances in __io_uring_submit(), this can
> eventually cause both io_uring_submit() and io_uring_sqe() return 0
> forever.
I'll look into that one.
> 2. There is also a related issue -- when using IORING_SETUP_SQPOLL, in
> case polling kernel thread already went to sleep(IORING_SQ_NEED_WAKEUP
> is set), io_uring_enter() just wakes it up and immediately reports all
> @to_submit requests are consumed, while this is not true until awaken
> thread will manage to handle them. At least this contradicts with man
> page, which states:
> > When the system call returns that a certain amount of SQEs have
> > been consumed and submitted, it's safe to reuse SQE entries in
> > the ring.
> It is easy to reproduce this bug -- just change e.g. ->offset
> field in the SQE immediately after io_uring_enter() successfully
> returns and you will see that IO happened on new offset.
Not sure how best to convery that bit of information. If you're using
the sq thread for submission, then we cannot reliably tell the
application when an sqe has been consumed. The application must look for
completions (successful or errors) in the CQ ring.
> 3. Again due to lack of synchronization between io_sq_thread() and
> io_uring_enter(), in case the ring is full and IORING_SETUP_SQPOLL is
> used, it seems there is no other way for application to wait for slots
> in SQ to become available but busy waiting for *sq->khead to advance.
> Thus from one busy waiting thread we get two. Is this the expected
> behavior? Should the user of IORING_SETUP_SQPOLL busy wait for slots
> in SQ?
You could wait on cq ring completions, each sqe should trigger one.
> 4. Minor one: in case sq_thread_idle is set to ridiculously big
> value(e.g. 100 sec), kernel watchdog starts reporting this as a bug.
> > Message from syslogd@centos-linux at Jun 21 20:00:04 ...
> > kernel:watchdog: BUG: soft lockup - CPU#0 stuck for 21s!
> > [io_uring-sq:10691]
Ah yes, cosmetic issue, I'll address that one as well.
--
Jens Axboe
> Can you try the attached patch and see if it fixes it for you?
Thank you very much, that worked like a charm for both O_DIRECT and page
cache. Below is the output for O_DIRECT reads submission on the same machine:
root@localhost:~/io_uring# ./io_uring_read_blkdev /dev/sda8
submitted_already = 0, submitted_now = 32, submit_time = 277 us
submitted_already = 32, submitted_now = 32, submit_time = 131 us
submitted_already = 64, submitted_now = 32, submit_time = 213 us
submitted_already = 96, submitted_now = 32, submit_time = 170 us
submitted_already = 128, submitted_now = 32, submit_time = 161 us
submitted_already = 160, submitted_now = 32, submit_time = 169 us
submitted_already = 192, submitted_now = 32, submit_time = 184 us
> Not sure how best to convery that bit of information. If you're using
> the sq thread for submission, then we cannot reliably tell the
> application when an sqe has been consumed. The application must look for
> completions (successful or errors) in the CQ ring.
I know that SQPOLL feature support is not fully implemented in liburing,
so for now it seems that io_uring_get_sqe() could return not actually
submitted SQE, editing which could lead to race between kernel polling
thread and user space. I just think it is worth mentioning this fact in
documentation.
> You could wait on cq ring completions, each sqe should trigger one.
Unfortunately few issues seem to arise if this approach is taken in
IO-intensive application. As a disclaimer I should note that SQ ring
overflow is a rare event given enough entries, nevertheless applications,
especially those using SQPOLL, should handle this situation gracefully
and in a performant manner.
So what we have is highly IO-intensive application that submits very
slow IOs*** (that's why it uses async IO in the first place) and
cares much about the progress of the submitting threads(the most probable
reason to use SQPOLL feature). Given such prerequisites, the following
scenario is probable:
*** by 'very slow' I mean IOs, completion of which takes significantly
more time than submission
1. Put @sq_entries with very slow IOs in SQ...
PENDING SQ INFLIGHT CQ
+---+ +---+ +---+ +---+---+
============>| X | | | | | |
+---+ +---+ +---+ +---+---+
...which will be submitted by polling thread
PENDING SQ INFLIGHT CQ
+---+ +---+ +---+ +---+---+
| | | |====>| X | | | |
+---+ +---+ +---+ +---+---+
2. Then try to add (@sq_entries + @pending) entries to SQ, but only
succeed with @sq_entries.
PENDING SQ INFLIGHT CQ
+---+ +---+ +---+ +---+---+
==>| X |====>| X | | X | | | |
+---+ +---+ +---+ +---+---+
3. Wait very long time in io_uring_enter(GETEVENTS) waiting for CQ ring
completion...
PENDING SQ INFLIGHT CQ
+---+ +---+ +---+ +---+---+
| X | | X | | |====>| X | |
+---+ +---+ +---+ +---+---+
...and still there is no guarantee that slot in SQ ring became
available. Should we call
io_uring_enter(GETEVENTS, min_complete = 1);
in a loop, checking (*khead == *ktail) at every iteration?
Concluding, it seems reasonable to instruct applications using SQPOLL to
submit SQEs until the queue is full, and then call io_uring_enter(),
probably with some flag, to wait for a slot in submission queue, not for
completions, since
1) Time needed to complete IO tends to be much greater than time needed
to submit it.
2) CQ ring completion does not imply the slot became available in SQ (see
diagram above).
3) Busy waiting of submitting thread is probably not what is desired by
SQPOLL users.
Side note: eventloop-driven applications could find themselves comforted
by epoll()-ing ioring fd with EPOLLOUT to wait for the available entry in
SQ. Do I understand it correctly that spurious wakeups are currently
possible since io_uring_poll() is awakened only on io_commit_cqring(),
which, as shown above, doesn't guarantee that EPOLLOUT may be set?
Thank you again!
__
Best regards,
Filipp Mikoian