RFR: 8358619: Fix interval recomputation in CPU Time Profiler

[Jaroslav Bachorik]

On Thu, 12 Jun 2025 09:27:30 GMT, Johannes Bechberger <jbechberger at openjdk.org> wrote:

> Fixes the recomputation issue by passing either the rate or the period directly to the sampler (instead of just the rate value with the period value converted into a rate). This prevents issues when the number of cores changes during the JFR recording.

src/hotspot/share/jfr/periodic/sampling/jfrCPUTimeThreadSampler.cpp line 171:

> 169:   double _rate;
> 170:   u8 _period_nanos;
> 171:   bool _is_rate;

Can this be turned into `union`? 
Only one of `_rate`  or `_period_nanos` will ever be set at a time.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25775#discussion_r2154393526