runtime: add g.p

[josharian]

Some important and hot data is stored in Ps, such as write barrier buffers and defer pools. Getting from a G to a P currently requires evaluating g.m.p. We should consider adding a direct link, g.p, to avoid an extra memory lookup. This should reduce memory lookups and cache pressure / cache misses on hot code, particularly write barrier writes.

This might also enable us to eliminate the mcache.

This requires updating g.p every time a G gets (un)scheduled.

I took a stab at this but never quite got it working. I hope someone else might.

cc @aclements @dvyukov

[dvyukov]

Sounds reasonable to try.
There are probably few hot things (like the ones required for GC barriers and malloc), so we could also try to cache them in g directly.

[josharian]

cc @benesch :)

[dvyukov]

I happened to write this code while debugging #27660 yesterday:
https://go-review.googlesource.com/c/go/+/148823/1/src/runtime/proc.go
It has all places for wiring and unwiring g and p marked. Feel free to productionize and submit.