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

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Message-ID: <c62a9f8a-312d-4f9e-9022-265e53564101@arm.com>
Date: Fri, 7 Mar 2025 20:28:13 +0000
From: Mark Barnett <mark.barnett@....com>
To: Leo Yan <leo.yan@....com>
Cc: peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
 namhyung@...nel.org, irogers@...gle.com, ben.gainey@....com,
 deepak.surti@....com, ak@...ux.intel.com, will@...nel.org,
 james.clark@....com, mark.rutland@....com,
 alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
 adrian.hunter@...el.com, linux-perf-users@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/5] perf: Allow periodic events to alternate between
 two sample periods

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.