Support stats "ratio_in_yjit"

[jethrodaniel]

What does this PR do?

When ruby's extended YJIT stats are available (--yjit-stat), then we have access to numerous additional stats.

This adds :ratio_in_yjit, which is only present when additional stats are enabled.

Example:

RUBYOPT='--yjit --yjit-stats=quiet' ruby -e '
  def foo = "a" * 10
  300.times{foo}
  puts "ratio_in_yjit: #{::RubyVM::YJIT.runtime_stats[:ratio_in_yjit]}"
'
ratio_in_yjit: 0.9550806290358625

Motivation:

This should allow for a better understanding of how to tune --yjit-exec-mem-size.

From https://github.com/ruby/ruby/blob/ef084cc8f4958c1b6e4ead99136631bef6d8ddba/doc/yjit/yjit.md?plain=1#L213-L223:

If you start Ruby with --yjit-stats, e.g. using an environment variable RUBYOPT=--yjit-stats,
RubyVM::YJIT.runtime_stats[:ratio_in_yjit] shows the ratio of YJIT-executed instructions in %.
Ideally, ratio_in_yjit should be as large as 99%, and increasing --yjit-exec-mem-size often
helps improving ratio_in_yjit.

Additional Notes:

Fwiw, I directly based this on the previous YJIT PRs, which were very helpful here:

• Gauge primitive RubyVM::YJIT.runtime_stats, if YJIT is enabled #2711
• Support stats "yjit_alloc_size" #3661

ratio_in_yjit has been available sine 3.3.0: https://github.com/ruby/ruby/blob/caf0d2058aa223515897401ff94e11e1c0671ce0/doc/NEWS/NEWS-3.3.0.md?plain=1#L436

How to test the change?

RUBYOPT="--yjit --yjit-stats=quiet" bundle exec rake spec:yjit

bundle exec rake spec:yjit # skipped

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 60.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 97.86%. Comparing base (58b17f0) to head (a06708a).
Report is 41 commits behind head on master.

Files with missing lines	Patch %	Lines
spec/datadog/core/runtime/metrics_spec.rb	63.63%	4 Missing ⚠️
lib/datadog/core/environment/yjit.rb	50.00%	1 Missing ⚠️
lib/datadog/core/runtime/metrics.rb	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3991      +/-   ##
==========================================
- Coverage   97.87%   97.86%   -0.01%     
==========================================
  Files        1314     1314              
  Lines       78652    78665      +13     
  Branches     3909     3911       +2     
==========================================
+ Hits        76978    76984       +6     
- Misses       1674     1681       +7     

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[ivoanjo]

Hey @jethrodaniel thanks for opening up the discussion on this! I think is a nice thing to add, would happily approve the PR once CI is happy and whatnot :)

[ivoanjo]

I've triggered the CI run and left a few notes. TBH I think this could go as-is, but I've suggested a few improvements ;)

[ivoanjo]

👍 LGTM. I realized my suggestion for the matrix did not address the part where 3.2 doesn't support --yjit-stats=quiet so I pushed a tiny change to address that.

I'm just waiting for CI to settle and I'll merge this in. Thanks for the nice contribution! 🙏