In order to implement the `full-pcpus-only` cpumanager policy option,
we leverage the implementation of the algorithm which picks CPUs.
By design, CPUs are taken from the biggest chunk available (socket
or NUMA zone) to physical cores, down to single cores.
Leveraging this, if the requested CPU count is a multiple of the SMT
level (commonly 2), we're guaranteed that only full physical cores
will be taken.
The hidden assumption here is this holds true by construction iff
the user reserved CPUs (if any) considering full physical CPUs.
IOW, if the user did intentionally or mistakely reserve single threads
which are no core siblings[1], then the simple check we implemented
is not sufficient.
A easy example can probably outline this better. With this setup:
cores: [(0, 4), (1, 5), (2, 6), (3, 8)] (in parens: thread siblings).
SMT level: 2 (each tuple is 2 elements)
Reserved CPUs: 0,1 (explicit pick using `--reserved-cpus`)
A container then requests 6 cpus. full-pcpus-only check: 6 % 2 == 0. Passed.
The CPU allocator will take first full cores, (2,6) and (3,8), and will
then pick the remaining single CPUs. The allocation will succeed, but
it's incorrect.
We can fix this case with a stricter precheck.
We need to additionally consider all the core siblings of the reserved
CPUs as unavailable when computing the free cpus, before to start the
actual allocation. Doing so, we fall back in the intended behavior, and
by construction all possible CPUs allocation whose number is multiple
of the SMT level are now correct again.
+++
[1] or thread siblings in the linux parlance, in any case:
hyperthread siblings of the same physical core
Signed-off-by: Francesco Romani <fromani@redhat.com>
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