Re: [PATCH v2 1/5] perf: Allow periodic events to alternate between two sample periods

[Mark Barnett]

On 1/21/25 13:01, Leo Yan wrote:
> >   	local64_set(&hwc->period_left, hwc->sample_period);
> >   
> > +	if (attr->alt_sample_period) {
> > +		hwc->sample_period = attr->alt_sample_period;
> > +		hwc->using_alt_sample_period = true;
> > +	}
>
> My understanding it sets a short sample window for the first period.
> Would it initialize the `hwc->period_left` with the updated sample
> period?

It sets the long period first: hwc->period_left is used to program the 
PMU when setting up the event, and hwc->sample_period is queued up as 
the next period to switch to.

> > +
> > +	/*
> > +	 * alt_sample_period cannot be used with freq
> > +	 */
> > +	if (attr->freq && attr->alt_sample_period)
> > +		goto err_ns;
> > +
>
> It is good to validate parameters first. So move the checking before
> the adjustment for the alt sample period.

Ack. Done.

> >   	/*
> >   	 * We do not support PERF_SAMPLE_READ on inherited events unless
> >   	 * PERF_SAMPLE_TID is also selected, which allows inherited events to
> > @@ -12763,9 +12788,19 @@ SYSCALL_DEFINE5(perf_event_open,
> >   	if (attr.freq) {
> >   		if (attr.sample_freq > sysctl_perf_event_sample_rate)
> >   			return -EINVAL;
> > +		if (attr.alt_sample_period)
> > +			return -EINVAL;
> >   	} else {
> >   		if (attr.sample_period & (1ULL << 63))
> >   			return -EINVAL;
> > +		if (attr.alt_sample_period) {
> > +			if (!attr.sample_period)
> > +				return -EINVAL;
> > +			if (attr.alt_sample_period & (1ULL << 63))
> > +				return -EINVAL;
> > +			if (attr.alt_sample_period == attr.sample_period)
> > +				attr.alt_sample_period = 0;
>
> In theory, the attr.alt_sample_period should be less than
> attr.sample_period, right?

Added some validation for this.