[PATCH 0/2] [FH:linux-azure] hv: vmbus: Fix duplicate CPU assignments within a device

Tim Gardner tim.gardner at canonical.com
Fri Aug 20 19:00:30 UTC 2021

BugLink: https://bugs.launchpad.net/bugs/1937078

SRU Justification


Description of issue and solution:

The vmbus module uses a rotational algorithm to assign target CPUs to
a device's channels. Depending on the timing of different device's channel
offers, different channels of a device may be assigned to the same CPU.

For example on a VM with 2 CPUs, if NIC A and B's channels are offered
in the following order, NIC A will have both channels on CPU0, and
NIC B will have both channels on CPU1 -- see below. This kind of
assignment causes RSS load that is spreading across different channels
to end up on the same CPU.

Timing of channel offers:
NIC A channel 0
NIC B channel 0
NIC A channel 1
NIC B channel 1

VMBUS ID 14: Class_ID = {f8615163-df3e-46c5-913f-f2d2f965ed0e} - Synthetic network adapter
Device_ID = {cab064cd-1f31-47d5-a8b4-9d57e320cccd}
Sysfs path: /sys/bus/vmbus/devices/cab064cd-1f31-47d5-a8b4-9d57e320cccd
Rel_ID=14, target_cpu=0
Rel_ID=17, target_cpu=0

VMBUS ID 16: Class_ID = {f8615163-df3e-46c5-913f-f2d2f965ed0e} - Synthetic network adapter
Device_ID = {244225ca-743e-4020-a17d-d7baa13d6cea}
Sysfs path: /sys/bus/vmbus/devices/244225ca-743e-4020-a17d-d7baa13d6cea
Rel_ID=16, target_cpu=1
Rel_ID=18, target_cpu=1

Update the vmbus CPU assignment algorithm to avoid duplicate CPU
assignments within a device.

The new algorithm iterates num_online_cpus + 1 times.
The existing rotational algorithm to find "next NUMA & CPU" is still here.
But if the resulting CPU is already used by the same device, it will try
the next CPU.
In the last iteration, it assigns the channel to the next available CPU
like the existing algorithm. This is not normally expected, because
during device probe, we limit the number of channels of a device to
be <= number of online CPUs.



[Test Plan]

Microsoft tested both patches. No regressions detected.
Performance criteria satisfied.

[Where problems could occur]

Network performance issues could persist.

[Other Info]

SF: #00315347

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