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fix: honor -stderrthreshold flag with klog v2 default -logtostderr=true#1677

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fix: honor -stderrthreshold flag with klog v2 default -logtostderr=true#1677
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@pierluigilenoci pierluigilenoci commented Apr 24, 2026

What this PR does

klog v2 defaults to -logtostderr=true, which silently causes the -stderrthreshold flag to be ignored. This is a long-standing issue tracked in kubernetes/klog#212.

klog v2.140.0 introduced the legacy_stderr_threshold_behavior flag to fix this behavior (kubernetes/klog#432).

Changes

  1. Bump k8s.io/klog/v2 from v2.130.1 to v2.140.0
  2. Set legacy_stderr_threshold_behavior=false in AddKlogFlags() (pkg/utils/global.go) so that -stderrthreshold is honored even when -logtostderr=true (the default)
  3. Set stderrthreshold=INFO as the default threshold, matching the expected behavior

Why

Without this fix, operators cannot use -stderrthreshold to filter log output by severity because klog's legacy behavior unconditionally copies everything to stderr when -logtostderr is true (which it is by default).

How to test

go build ./pkg/utils/

The only klog.InitFlags call in non-vendor, non-test code is in pkg/utils/global.go, which is where the fix is applied.

Note

This PR was created with the assistance of AI tooling (Claude Code).

@pierluigilenoci
fix: honor -stderrthreshold flag with klog v2 default -logtostderr=true
53b9967 
klog v2 defaults to -logtostderr=true, which silently causes
-stderrthreshold to be ignored. This is a long-standing issue
(kubernetes/klog#212) fixed in klog v2.140.0 via the new
legacy_stderr_threshold_behavior flag (kubernetes/klog#432).

Bump k8s.io/klog/v2 from v2.130.1 to v2.140.0 and set
legacy_stderr_threshold_behavior=false in AddKlogFlags so that
-stderrthreshold works as expected.

Signed-off-by: Pierluigi Lenoci <pierluigi.lenoci@gmail.com>
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k8s-ci-robot commented Apr 24, 2026

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@k8s-ci-robot k8s-ci-robot added the do-not-merge/work-in-progress Indicates that a PR should not merge because it is a work in progress. label Apr 24, 2026
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[APPROVALNOTIFIER] This PR is NOT APPROVED

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@k8s-ci-robot k8s-ci-robot requested review from huww98 and mowangdk April 24, 2026 08:03
@k8s-ci-robot k8s-ci-robot added size/XL Denotes a PR that changes 500-999 lines, ignoring generated files. cncf-cla: yes Indicates the PR's author has signed the CNCF CLA. labels Apr 24, 2026
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huww98 commented Apr 24, 2026

How is this related to -v flag?

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pierluigilenoci commented Apr 24, 2026

Good question! The -v flag and -stderrthreshold are both klog flags registered by klog.InitFlags(), but they serve different purposes:

  • -v controls the verbosity level (which log messages are generated)
  • -stderrthreshold controls which severity levels (INFO, WARNING, ERROR, FATAL) are sent to stderr

They are related because both are part of the same klog flag set initialized in AddKlogFlags(), and an operator might reasonably set both (e.g., -v=4 -stderrthreshold=WARNING to generate verbose logs but only send warnings and errors to stderr).

The bug (kubernetes/klog#212) is that -stderrthreshold is silently ignored when -logtostderr=true (the default). This fix opts into the corrected behavior so -stderrthreshold actually works as documented. It does not change -v behavior at all — it just fixes -stderrthreshold so operators can filter which severity levels reach stderr, regardless of the verbosity level set via -v.

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pierluigilenoci commented Apr 26, 2026

Good question @huww98. They're orthogonal:

  • -v (verbosity): Controls which log messages are generated. -v=3 means klog.V(3) messages are produced; higher levels are suppressed entirely.
  • -stderrthreshold (severity threshold): Controls which severity levels are written to stderr. For example, stderrthreshold=WARNING means only WARNING and ERROR messages appear on stderr; INFO messages are suppressed from stderr output.

The bug this PR fixes: klog v2 defaults to logtostderr=true with a legacy behavior mode that silently ignores -stderrthreshold. So even if a user passes --stderrthreshold=ERROR, all log levels still appear on stderr. The fix opts out of the legacy behavior so -stderrthreshold actually works as documented.

See kubernetes/klog#432 for the upstream context.

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huww98 commented Apr 27, 2026 •
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OK. But I think we don't care this much.
As per https://github.com/kubernetes/enhancements/tree/master/keps/sig-instrumentation/2845-deprecate-klog-specific-flags-in-k8s-components . Most kubernetes components only supports output to stderr now. and already deprecated stderrthreshold flag. For this project, I haven't seen anyone using log file.

WARNING severity level even not exist in the new structured logging API. We now only have ERROR and INFO severity level, making stderrthreshold not very useful.

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pierluigilenoci commented Apr 27, 2026

@huww98 You're right — with KEP-2845 deprecating klog-specific flags and the move to structured logging, the stderrthreshold fix doesn't add practical value here. Thanks for the explanation and for taking the time to review!

Closing this PR.

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