This check can be simplified to

if (_sampling_interval != sampling_interval) {
       _sampling_interval = sampling_interval;
       _last_sample_time = 0;
       _process_history.clear();
}

Please note that the current logic doesn't really protect against stale data,

• start,cpu,proc=30
• Profile for 2 minutes
• stop
• No profiles for for 5 minutes
• start,cpu,proc=30

The _process_history won't be cleared even by all accounts it should be cleared as it contains stale data
I would recommend changing the condition to something along the following lines

if (_sampling_interval != sampling_interval || (_next_sample_time + sampling_interval) < wall_time) {
       _sampling_interval = sampling_interval;
       _last_sample_time = 0;
       _process_history.clear();
}

• _sampling_interval != sampling_interval checks for a change in the interval
• (_next_sample_time + sampling_interval) < wall_time checks if the profiler was turned off for an extended duration

Note: In the check above I'm checking for exactly one missing sample as an example, but 1 missing sample might be fine, I would recommend checking a proper safety margin at which we should consider the data in our _process_history stale & clear it.

This condition looks suspicious indeed, since _next_sample_time is always _last_sample_time + _sampling_interval.

@apangin what are your thoughts about the stale data issue?

As described here #1411 (comment)
previously the loop option was indirectly modifying the sampling frequency, this was fixed in recent changes,

The fix basically converted to the engine to a static one with static data which are preserved between profiling sessions as to bypass the loop restart.

If the user uses adhoc profiling via asprof, it's quite reasonable to assume the profiler might have a long lag between profiling session.

So what should be an acceptable threshold to keep using the old historical data from previous profiling session?

This condition looks suspicious indeed, since _next_sample_time is always _last_sample_time + _sampling_interval.