num_cpus::get()=0 on mipsel-unknown-linux-gnu

[kpp]

See tokio-rs/tokio#400

[kpp]

ping

[seanmonstar]

I know nothing about mipsel.

PRs welcome.

[lnicola]

The seems to be missing a failsafe check for Linux in order not to return less than 1.

[seanmonstar]

Looks like there is a check, but not in the sched affinity branch.

[lnicola]

See #68.

[seanmonstar]

I think there's still a behavior question that needs answering for #68: should it fallback to just the count? Or should an affinity count of 0 mean "you aren't assigned any CPUs, so here, have just 1"?

[lnicola]

Does an affinity mask of 0 mean that the process is not running?

[lnicola]

On my system, sched_setaffinity with a mask of 0 returns EINVAL, which makes sense. So if the sched_getaffinity returns 0, then the affinity mask is broken on that system and the process shouldn't be running anyway. So we get no useful information, and we should handle that case as if sched_getaffinity returned an error.

I don't think returning 1 in this case is useful. We should fall back to sysconf.

Hmm, I actually have a mips (not mipsel though) device, but I don't know how to link with the C library there (uClibc?).

[seanmonstar]

I've been trying to research what others do in this case:

• In OpenJDK, if sched_getaffinity(args) == 0, it has an assert(count > 0) and just returns it afterwards. So, seems like it would crash there?
• In Golang, if the affinity mask has 0 set, it explicitly returns 1.

[lnicola]

If that's the C assert (the #include is missing), it crashes in "debug" builds and has no effect in release (NDEBUG) ones.

I don't really expect a lot of software to handle this case. Maybe the issue is a different one?

Also, this sounds fun:

The CPU affinity system calls were introduced in Linux kernel 2.5.8. The system call wrappers were introduced in glibc 2.3. Initially, the glibc interfaces included a cpusetsize argument, typed as
unsigned int. In glibc 2.3.3, the cpusetsize argument was removed, but was then restored in glibc 2.3.4, with type size_t.