[PATCHSET RFC v2 0/4] uring_cmd copy avoidance

[Jens Axboe <axboe@kernel.dk>]

Hi,

Currently uring_cmd unconditionally copies the SQE at prep time, as it
has no other choice - the SQE data must remain stable after submit.
This can lead to excessive memory bandwidth being used for that copy,
as passthrough will often use 128b SQEs, and efficiency concerns as
those copies will potentially use quite a lot of CPU cycles as well.

As a quick test, running the current -git kernel on a box with 23
NVMe drives doing passthrough IO, memcpy() is the highest cycle user
at 9.05%, which is all off the uring_cmd prep path. The test case is
a 512b random read, which runs at 91-92M IOPS.

With these patches, memcpy() is gone from the profiles, and it runs
at 98-99M IOPS, or about 7-8% faster.

Before:

IOPS=91.12M, BW=44.49GiB/s, IOS/call=32/32
IOPS=91.16M, BW=44.51GiB/s, IOS/call=32/32
IOPS=91.18M, BW=44.52GiB/s, IOS/call=31/32
IOPS=91.92M, BW=44.88GiB/s, IOS/call=32/32
IOPS=91.88M, BW=44.86GiB/s, IOS/call=32/32
IOPS=91.82M, BW=44.83GiB/s, IOS/call=32/31
IOPS=91.52M, BW=44.69GiB/s, IOS/call=32/32

with the top perf report -g --no-children being:

+    9.07%  io_uring  [kernel.kallsyms]  [k] memcpy

and after:

# bash run-peak-pass.sh
[...]
IOPS=99.30M, BW=48.49GiB/s, IOS/call=32/32
IOPS=99.27M, BW=48.47GiB/s, IOS/call=31/32
IOPS=99.60M, BW=48.63GiB/s, IOS/call=32/32
IOPS=99.68M, BW=48.67GiB/s, IOS/call=32/31
IOPS=99.80M, BW=48.73GiB/s, IOS/call=31/32
IOPS=99.84M, BW=48.75GiB/s, IOS/call=32/32

with memcpy not even in profiles. If you do the actual math of 100M
requests per second, and 128b of copying per IOP, then it's almost
12GB/sec of reduced memory bandwidth.

Even for lower IOPS production testing, Caleb reports that memcpy()
overhead is in the realm of 1.1% of CPU time.

v2 of this patchset takes a different approach than v1 did - rather
than have the core mark a request as being potentially issued
out-of-line, this one adds an io_cold_def ->sqe_copy() helper, and
puts the onus on io_uring core to call it appropriately. Outside of
that, it also adds an IO_URING_F_INLINE flag so that the copy helper
_knows_ if it may sanely copy the SQE, or whether there's a bug in
the core and it should just be ended with -EFAULT. Where possible,
the actual SQE is also passed in.

I think this approach is saner, and in fact it can be extended to
reduce over-eager copies in other spots. For now I just did uring_cmd,
and verified that the memcpy's are still gone from my test.

Can also be found here:

https://git.kernel.dk/cgit/linux/log/?h=uring_cmd.2

 include/linux/io_uring_types.h |  2 ++
 io_uring/io_uring.c            | 35 +++++++++++++++------
 io_uring/opdef.c               |  1 +
 io_uring/opdef.h               |  1 +
 io_uring/uring_cmd.c           | 57 ++++++++++++++++++----------------
 io_uring/uring_cmd.h           |  2 ++
 6 files changed, 63 insertions(+), 35 deletions(-)

-- 
Jens Axboe