linux-kernel - Re: [PATCH 2/3] x86, perf: Add a separate Arch Perfmon v4 PMI handler

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Date:   Tue, 7 Aug 2018 11:29:54 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, mingo@...hat.com, acme@...nel.org,
        linux-kernel@...r.kernel.org, eranian@...gle.com,
        ak@...ux.intel.com, alexander.shishkin@...ux.intel.com
Subject: Re: [PATCH 2/3] x86, perf: Add a separate Arch Perfmon v4 PMI handler



On 8/6/2018 2:35 PM, Peter Zijlstra wrote:
> On Mon, Aug 06, 2018 at 10:23:42AM -0700, kan.liang@...ux.intel.com wrote:
>> @@ -2044,6 +2056,14 @@ static void intel_pmu_disable_event(struct perf_event *event)
>>   	if (unlikely(event->attr.precise_ip))
>>   		intel_pmu_pebs_disable(event);
>>   
>> +	/*
>> +	 * We could disable freezing here, but doesn't hurt if it's on.
>> +	 * perf remembers the state, and someone else will likely
>> +	 * reinitialize.
>> +	 *
>> +	 * This avoids an extra MSR write in many situations.
>> +	 */
>> +
>>   	if (unlikely(hwc->config_base == MSR_ARCH_PERFMON_FIXED_CTR_CTRL)) {
>>   		intel_pmu_disable_fixed(hwc);
>>   		return;
>> @@ -2119,6 +2139,11 @@ static void intel_pmu_enable_event(struct perf_event *event)
>>   	if (event->attr.exclude_guest)
>>   		cpuc->intel_ctrl_host_mask |= (1ull << hwc->idx);
>>   
>> +	if (x86_pmu.counter_freezing && !cpuc->frozen_enabled) {
>> +		enable_counter_freeze();
>> +		cpuc->frozen_enabled = 1;
>> +	}
>> +
>>   	if (unlikely(event_is_checkpointed(event)))
>>   		cpuc->intel_cp_status |= (1ull << hwc->idx);
>>   
> 
> Why here? That doesn't really make sense; should this not be in
> intel_pmu_cpu_starting() or something?


For Goldmont Plus, the counter freezing feature can be re-enabled at 
run-time by loading a newer microcode.
We need to check the x86_pmu.counter_freezing every time.


> 
>> +static bool disable_counter_freezing;
>> +module_param(disable_counter_freezing, bool, 0444);
>> +MODULE_PARM_DESC(disable_counter_freezing, "Disable counter freezing feature."
>> +		"The PMI handler will fall back to generic handler."
>> +		"Default is false (enable counter freezing feature).");
> 
> Why?
> 
>> +	/*
>> +	 * Ack the PMU late after the APIC.  This avoids bogus
> 
> That doesn't make sense. PMU and APIC do not have order.
>

The moment FROZEN bit is cleared, counters start counting immediately.
Here, we try to make it as close as possible to IRET.


Thanks,
Kan