build: Add script for pgo/lto builds

[hodgesds]

Add a script to generate PGO/LTO builds. This may improve performance for some of the more userspace oriented schedulers. See this section in the Rust book on PGO. In nightly versions of cargo I think we should be able to get rid of the script and add a build profile (issue).

If everything goes right and the toolchain supports it then a binary should be produced:

$ ./pgo-lto.sh /tmp/pgo-data scx_rustland 10
<lots of output>
   Compiling ordered-float v3.9.2
   Compiling simplelog v0.12.2
   Compiling scx_rustland_core v2.2.8 (/root/scx/rust/scx_rustland_core)
   Compiling fb_procfs v0.7.1
   Compiling scx_rustland v1.0.9 (/root/scx/scheds/rust/scx_rustland)
warning: scx_rustland_core.307c2d1e9a16060e-cgu.0: function control flow change detected (hash mismatch) __rust_realloc Hash = 2202711501531239 up to 467244 count discarded

warning: scx_rustland_core.307c2d1e9a16060e-cgu.0: function control flow change detected (hash mismatch) __rust_alloc_zeroed Hash = 2202711501531239 up to 198 count discarded

warning: `scx_rustland_core` (lib) generated 2 warnings
    Finished `release` profile [optimized] target(s) in 1m 59s

[JakeHillion]

I don't see any reason not to do this, though I think to get the most benefit you'd have to run a realistic workload instead of the sleep "$PGO_DUR". This one might end up optimising some of the early returns instead of the important bits if the machine is quiet for example.

Maybe we should throw this in a directory like build? Not sure if that name is already taken.

[etsal]

Seems reasonable to me!

I don't see any reason not to do this, though I think to get the most benefit you'd have to run a realistic workload instead of the sleep "$PGO_DUR". This one might end up optimising some of the early returns instead of the important bits if the machine is quiet for example.

Given that most schedulers mostly do a bunch of initialization at startup then a bunch of periodic jobs in userspace that wouldn't be that big of an issue, e.g., scx_rusty still makes an attempt to load balance and traverses the relevant code paths even if the machine is relatively idle.