[PATCHv7 RFC 0/3] Add visibility for native NVMe multipath using sysfs

[Nilay Shroff <nilay@linux.ibm.com>]

Hi,

This RFC propose adding new sysfs attributes for adding visibility of
nvme native multipath I/O.

The changes are divided into three patches.
The first patch adds visibility for round-robin io-policy.
The second patch adds visibility for numa io-policy.
The third patch adds the visibility for queue-depth io-policy.

As we know, NVMe native multipath supports three different io policies
(numa, round-robin and queue-depth) for selecting I/O path, however, we
don't have any visibility about which path is being selected by multipath
code for forwarding I/O. This RFC helps add that visibility by adding new
sysfs attribute files named "numa_nodes" and "queue_depth" under each
namespace block device path /sys/block/nvmeXcYnZ/. We also create a
"multipath" sysfs directory under head disk node and then from this
directory add a link to each namespace path device this head disk node
points to.

Please find below output generated with this proposed RFC patch applied on
a system with two multi-controller PCIe NVMe disks attached to it. This
system is also an NVMf-TCP host which is connected to an NVMf-TCP target
over two NIC cards. This system has four numa nodes online when the below
output was captured:

# cat /sys/devices/system/node/online
0-3

# lscpu
<snip>
NUMA:
  NUMA node(s):           4
  NUMA node0 CPU(s):
  NUMA node1 CPU(s):      0-7
  NUMA node2 CPU(s):      8-31
  NUMA node3 CPU(s):      32-63
<snip>

Please note that numa node 0 though online, doesn't have any CPU
currently assigned to it.

# nvme list -v
Subsystem        Subsystem-NQN                                                                                    Controllers
---------------- ------------------------------------------------------------------------------------------------ ----------------
nvme-subsys1     nqn.1994-11.com.samsung:nvme:PM1735a:2.5-inch:S6RTNE0R900057                                     nvme0, nvme1
nvme-subsys3     nqn.2019-10.com.kioxia:KCM7DRUG1T92:3D60A04906N1                                                 nvme2, nvme3
nvme-subsys4     nvmet_subsystem                                                                                  nvme4, nvme5

Device           Cntlid SN                   MN                                       FR       TxPort Address        Slot   Subsystem    Namespaces
---------------- ------ -------------------- ---------------------------------------- -------- ------ -------------- ------ ------------ ----------------
nvme0    66     S6RTNE0R900057       3.2TB NVMe Gen4 U.2 SSD III              REV.SN66 pcie   052e:78:00.0   U50EE.001.WZS000E-P3-C14-R1 nvme-subsys1 nvme1n1
nvme1    65     S6RTNE0R900057       3.2TB NVMe Gen4 U.2 SSD III              REV.SN66 pcie   058e:78:00.0   U50EE.001.WZS000E-P3-C14-R2 nvme-subsys1 nvme1n1
nvme2    2      3D60A04906N1         1.6TB NVMe Gen4 U.2 SSD IV               REV.CAS2 pcie   0524:28:00.0   U50EE.001.WZS000E-P3-C4-R1 nvme-subsys3 nvme3n1
nvme3    1      3D60A04906N1         1.6TB NVMe Gen4 U.2 SSD IV               REV.CAS2 pcie   0584:28:00.0   U50EE.001.WZS000E-P3-C4-R2 nvme-subsys3 nvme3n1
nvme4    1      a224673364d1dcb6fab9 Linux                                    6.9.0-rc tcp    traddr=20.0.0.200,trsvcid=4420,src_addr=20.0.0.100        nvme-subsys4 nvme4n1
nvme5    2      a224673364d1dcb6fab9 Linux                                    6.9.0-rc tcp    traddr=10.0.0.200,trsvcid=4420,src_addr=10.0.0.100        nvme-subsys4 nvme4n1

Device            Generic           NSID       Usage                      Format           Controllers
----------------- ----------------- ---------- -------------------------- ---------------- ----------------
/dev/nvme1n1 /dev/ng1n1   0x1          5.75  GB /   5.75  GB      4 KiB +  0 B   nvme0, nvme1
/dev/nvme3n1 /dev/ng3n1   0x2          0.00   B /   5.75  GB      4 KiB +  0 B   nvme2, nvme3
/dev/nvme4n1 /dev/ng4n1   0x1          5.75  GB /   5.75  GB      4 KiB +  0 B   nvme4, nvme5


# nvme show-topology
nvme-subsys1 - NQN=nqn.1994-11.com.samsung:nvme:PM1735a:2.5-inch:S6RTNE0R900057
               hostnqn=nqn.2014-08.org.nvmexpress:uuid:41528538-e8ad-4eaf-84a7-9c552917d988
               iopolicy=numa
\
 +- ns 1
 \
  +- nvme0 pcie 052e:78:00.0 live optimized
  +- nvme1 pcie 058e:78:00.0 live optimized

nvme-subsys3 - NQN=nqn.2019-10.com.kioxia:KCM7DRUG1T92:3D60A04906N1
               hostnqn=nqn.2014-08.org.nvmexpress:uuid:41528538-e8ad-4eaf-84a7-9c552917d988
               iopolicy=round-robin
\
 +- ns 2
 \
  +- nvme2 pcie 0524:28:00.0 live optimized
  +- nvme3 pcie 0584:28:00.0 live optimized

nvme-subsys4 - NQN=nvmet_subsystem
               hostnqn=nqn.2014-08.org.nvmexpress:uuid:41528538-e8ad-4eaf-84a7-9c552917d988
               iopolicy=queue-depth
\
 +- ns 1
 \
  +- nvme4 tcp traddr=20.0.0.200,trsvcid=4420,src_addr=20.0.0.100 live optimized
  +- nvme5 tcp traddr=10.0.0.200,trsvcid=4420,src_addr=10.0.0.100 live optimized

As we could see above, we've three shared namespaces created. In terms of
iopolicy, we have "numa" configured for nvme-subsys1, "round-robin"
configured for nvme-subsys3 and "queue-depth" configured for nvme-subsys4.

Now, under each namespace "head disk node", we create a sysfs group
attribute named "multipath". The "multipath" group then points to the
each path this head disk node points to:

# tree /sys/block/nvme1n1/multipath/
/sys/block/nvme1n1/multipath/
├── nvme1c0n1 -> ../../../../../pci052e:78/052e:78:00.0/nvme/nvme0/nvme1c0n1
└── nvme1c1n1 -> ../../../../../pci058e:78/058e:78:00.0/nvme/nvme1/nvme1c1n1

# tree /sys/block/nvme3n1/multipath/
/sys/block/nvme3n1/multipath/
├── nvme3c2n1 -> ../../../../../pci0524:28/0524:28:00.0/nvme/nvme2/nvme3c2n1
└── nvme3c3n1 -> ../../../../../pci0584:28/0584:28:00.0/nvme/nvme3/nvme3c3n1

# tree /sys/block/nvme4n1/multipath/
/sys/block/nvme4n1/multipath/
├── nvme4c4n1 -> ../../../../nvme-fabrics/ctl/nvme4/nvme4c4n1
└── nvme4c5n1 -> ../../../../nvme-fabrics/ctl/nvme5/nvme4c5n1

One can easily infer from the above output that for the "round-robin"
I/O policy, configured under nvme-subsys3, the I/O workload targeted at
nvme3n1 would toggle across nvme3c2n1 and nvme3c3n1 assuming the ana state
of each path is optimized (as can be seen in the output of show-topology).

For numa I/O policy, configured under nvme-subsys1, the "numa_nodes"
attribute file shows the numa nodes being preferred by the respective
namespace path. The numa nodes value is comma delimited list of nodes or
A-B range of nodes.

# cat  /sys/block/nvme1n1/multipath/nvme1c0n1/numa_nodes
0-1

# cat  /sys/block/nvme1n1/multipath/nvme1c1n1/numa_nodes
2-3

From the above output, one can easily infer that I/O workload targeted at
nvme1n1 and running on numa nodes 0 and 1 would use path nvme1c0n1.
Similarly, I/O workload running on numa nodes 2 and 3 would use path
nvme1c1n1.

For queue-depth I/O policy, configured under nvme-subsys4, the "queue_depth"
attribute file shows the number of active/in-flight I/O requests currently
queued for each path.

# cat  /sys/block/nvme4n1/multipath/nvme4c4n1/queue_depth
518

# cat  /sys/block/nvme4n1/multipath/nvme4c5n1/queue_depth
504

From the above output, one can easily infer that I/O workload targeted at
nvme4n1 uses two paths nvme4c4n1 and nvme4c5n1 and the current queue depth
of each path is 518 and 504 respectively.

Changes since v6:
    - Fix sysfs link warning menifested while running blktest nvme/058
	  (Keith Busch, Chaitanya Kulkarni)
    - Link to v6: https://lore.kernel.org/all/20241213041908.1381196-1-nilay@linux.ibm.com/
Changes since v5:
    - Fix typo in the subject line of the first patch in the series (Danied
      Wagner)
    - Link to v5: https://lore.kernel.org/all/20241030104156.747675-1-nilay@linux.ibm.com/
changes since v4:
    - Ensure that we create sysfs link from head gendisk node to each path
      device irrespective of the ANA state of the path (Hannes Reinecke)
    - Split the patch into three patch series and add commentary in the
      code so that it's easy to read and understand the core logic (Sagi
      Grimberg)
    - Don't show any output if user reads "numa_nodes" file and configured
      iopolicy is anything but numa; similarly don't emit any output if user
      reads "queue_depth" file and configured iopolicy is anything but
      queue-depth (Sagi Grimberg)
    - Link to v4: https://lore.kernel.org/all/20240911062653.1060056-1-nilay@linux.ibm.com/

Changes since v3:
    - Protect the namespace dereference code with srcu read lock (Daniel Wagner)
    - Link to v3: https://lore.kernel.org/all/20240903135228.283820-1-nilay@linux.ibm.com/

Changes since v2:
    - Use one value per one sysfs attribute (Keith Busch)
    - Link to v2: https://lore.kernel.org/all/20240809173030.2281021-1-nilay@linux.ibm.com/

Changes since v1:
    - Use sysfs to export multipath I/O information instead of debugfs
    - Link to v1: https://lore.kernel.org/all/20240722093124.42581-1-nilay@linux.ibm.com/

Nilay Shroff (3):
  nvme-multipath: Add visibility for round-robin io-policy
  nvme-multipath: Add visibility for numa io-policy
  nvme-multipath: Add visibility for queue-depth io-policy

 drivers/nvme/host/core.c      |   3 +
 drivers/nvme/host/multipath.c | 138 ++++++++++++++++++++++++++++++++++
 drivers/nvme/host/nvme.h      |  20 ++++-
 drivers/nvme/host/sysfs.c     |  20 +++++
 4 files changed, 177 insertions(+), 4 deletions(-)

-- 
2.47.1