TL;DR; is there any way a userspace thread in FreeBSD can tell
on which CPU it is (was) running ? I know the thread can migrate
at any time but as long as the event is rare I can live with
the occasionally wrong information.
Linux has getcpu(2) which is exposed by glibc as sched_getcpu(3),
but the linuxulator in FreeBSD only has a dummy (uniplemented)
function for that syscall.

It is common practice, when building scalable systems, to use per-cpu
resources that can be accessed without contention by just protecting
them with a CLI; STI; pair. Multiqueue NICs typically do this to
build a lock-free transmit path. In [1] we show an in-kernel
scheduler that maps a large number of clients to a (much smaller)
number of lock-free mailboxes, one per core.

In the kernel we can do CLI and STI and access curcpu.
In userspace a suitably privileged process can indeed open /dev/io
to acquire the right to use CLI and STI, though I am not sure
wether curcpu is available in some way.

Of course running userspace code with interrupts disabled is risky,
but we can use the per-cpu trick with a small tweak, namely,
protect each resouce with a lock. If the thread does not migrate
imediately after getcpu(), we will access the lock (and the resource)
almost always from the same cpu hence very efficiently.
Occasional migration may cause contention but should not
alter too much the good performance of this scheme.

So, any idea before I add a syscall/ioctl (or extend one)
to export this information ?


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