Add a jobserver proxy to ensure at least one token is always held

[Zoxc]

This adds a jobserver proxy to ensure at least one token is always held by rustc. Currently with -Z threads rustc can temporarily give up all its tokens, causing cargo to spawn additional rustc instances beyond the job limit.

The current behavior causes an issue with cargo fix which has a global lock preventing concurrent rustc instances, but it also holds a jobserver token, causing a deadlock when rustc gives up its token. That is fixed by this PR.

Fixes #133873 and #140093.

[rustbot]

r? @jieyouxu

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[jieyouxu]

r? @SparrowLii

[SparrowLii]

cc @matthiaskrgr

[SparrowLii]

Thanks!
@bors try @rust-timer queue

[bors]

⌛ Trying commit 141055f with merge c4cd906f6dc68c4343777d06a86285dbccb53e58...

[SparrowLii]

We can add a comment to explain that another thread was released so we don't need the new thread.

[Zoxc]

I don't follow your comment here.

[bors]

☀️ Try build successful - checks-actions
Build commit: c4cd906 (c4cd906f6dc68c4343777d06a86285dbccb53e58)

[bjorn3]

cg_clif has https://github.com/rust-lang/rustc_codegen_cranelift/blob/master/src/concurrency_limiter.rs which I think does something similar to this PR.

[Zoxc]

@bjorn3 What's the purpose of ConcurrencyLimiter there? It's a bit unclear to me.

[rust-timer]

Finished benchmarking commit (c4cd906): comparison URL.

Overall result: ❌ regressions - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.4%	[0.4%, 0.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -0.7%, secondary -2.2%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.1%	[0.9%, 1.2%]	2
Regressions ❌ (secondary)	1.8%	[1.8%, 1.8%]	1
Improvements ✅ (primary)	-0.9%	[-1.7%, -0.5%]	12
Improvements ✅ (secondary)	-2.9%	[-4.6%, -1.9%]	5
All ❌✅ (primary)	-0.7%	[-1.7%, 1.2%]	14

Cycles

Results (primary -0.5%, secondary -2.8%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.5%	[-0.5%, -0.4%]	2
Improvements ✅ (secondary)	-2.8%	[-3.4%, -2.2%]	12
All ❌✅ (primary)	-0.5%	[-0.5%, -0.4%]	2

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 775.257s -> 775.095s (-0.02%)
Artifact size: 365.06 MiB -> 365.08 MiB (0.01%)

[bjorn3]

@bjorn3 What's the purpose of ConcurrencyLimiter there? It's a bit unclear to me.

It is responsible for requesting jobserver tokens and ensuring that the implicit token isn't returned to the jobserver. It also caches tokens that are expected to be used soon (it gets the total amount of cgus and only returns tokens if the amount of remaining cgus is less than the amount of tokens the compilation session has claimed in total).

[Zoxc]

I changed the code to use the jobserver's helper thread to help deal with a race which could leak a jobserver token when exiting the process.