[PATCH v3 3/5] perf: Allow adding fixed random jitter to the alternate sampling period

[Peter Zijlstra]

On Tue, Mar 11, 2025 at 12:31:29PM +0100, Peter Zijlstra wrote:
> +	sample_period = hwc->sample_period_base;
> +	if (hwc->state & PERF_HES_HF_RAND) {
> +		u64 rand = 1 << event->attr.hf_sample_rand;
> +		sample_period -= rand / 2;
> +		sample_period += prandom_u32_state(&perf_rand) & (rand - 1);
> +	}
> +	if (hwc->state & PERF_HES_HF_ON) {
> +		u64 hf_sample_period = event->attr.hf_sample_period;
> +
> +		if (sample_period < hf_sample_period) {
> +			hwc->state &= ~PERF_HES_HF_ON;
> +			goto set_period;
> +		}
> +
> +		if (!(hwc->state & PERF_HES_HF_SAMPLE)) {
> +			hwc->sample_period -= hf_sample_period;
> +			hwc->state |= PERF_HES_HF_SAMPLE;
> +		} else {
> +			hwc->sample_period = hf_sample_period;
> +			hwc->state &= ~PERF_HES_HF_SAMPLE;

and obviously this should be the local sample_period modified above, not
the hwc one.

> +		}
> +	}
> +set_period:
> +	hwc->sample_period = sample_period;