runtime: GC should wake up idle Ps

[aclements]

Currently the GC doesn't always wake up idle Ps, and hence may not take full advantage of idle marking during the concurrent mark phase. This can happen during mark 2 because mark 1 completion preempts all workers; if the Ps running those workers have nothing else to do they will simply park, and there's no mechanism to wake them up after we allow mark workers to start again. It's possible this can happen during mark 1 as well, though it may be since we allow mark workers to run before starting the world for mark 1 that all of the Ps start running.

/cc @RLH

[rsc]

ping @aclements for 1.7 vs 1.8 triage

[aclements]

Not a serious issue, so moving to 1.8.

[aclements]

This is in fact an issue during mark 1, but for a different reason: I believe we do always start the idle workers for mark 1, but it looks like they can sometimes run out of work very early and exit, causing the Ps to go idle again. We don't wake those up once more work is enqueued, so they stay idle.

[quentinmit]

@aclements Are we still on track to do this for Go1.8?

[gopherbot]

CL https://golang.org/cl/32434 mentions this issue.