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From: "Liang, Kan" <kan.liang@linux.intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: acme@kernel.org, mingo@redhat.com, linux-kernel@vger.kernel.org,
	tglx@linutronix.de, jolsa@kernel.org, eranian@google.com,
	alexander.shishkin@linux.intel.com, ak@linux.intel.com
Subject: Re: [PATCH 4/9] perf/x86/intel: Support hardware TopDown metrics
Date: Tue, 28 May 2019 14:23:09 -0400	[thread overview]
Message-ID: <287c2c84-25cf-fdce-a3c3-49a6ee93ae4c@linux.intel.com> (raw)
In-Reply-To: <20190528124349.GU2606@hirez.programming.kicks-ass.net>



On 5/28/2019 8:43 AM, Peter Zijlstra wrote:
> On Tue, May 21, 2019 at 02:40:50PM -0700, kan.liang@linux.intel.com wrote:
>> The 8bit metrics ratio values lose precision when the measurement period
>> gets longer.
>>
>> To avoid this we always reset the metric value when reading, as we
>> already accumulate the count in the perf count value.
>>
>> For a long period read, low precision is acceptable.
>> For a short period read, the register will be reset often enough that it
>> is not a problem.
> 
>> The PERF_METRICS may report wrong value if its delta was less than 1/255
>> of SLOTS (Fixed counter 3).
>>
>> To avoid this, the PERF_METRICS and SLOTS registers have to be reset
>> simultaneously. The slots value has to be cached as well.
> 
> That doesn't sound like it is NMI-safe.
>  >
> 
>> RDPMC
>> =========
>> The TopDown can be collected per thread/process. To use TopDown
>> through RDPMC in applications on Icelake, the metrics and slots values
>> have to be saved/restored during context switching.
>>
>> Add specific set_period() to specially handle the slots and metrics
>> event. Because,
>>   - The initial value must be 0.
>>   - Only need to restore the value in context switch. For other cases,
>>     the counters have been cleared after read.
> 
> So the above claims to explain RDPMC, but doesn't mention that magic
> value below at all. In fact, I don't see how the above relates to RDPMC
> at all.

Current perf only support per-core Topdown RDPMC. On Icelake, it can be 
extended to per-thread Topdown RDPMC.
It tries to explain the extra work for per-thread topdown RDPMC, e.g. 
save/restore slots and metrics value in context switch.


> 
>> @@ -2141,7 +2157,9 @@ static int x86_pmu_event_idx(struct perf_event *event)
>>   	if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED))
>>   		return 0;
>>   
>> -	if (x86_pmu.num_counters_fixed && idx >= INTEL_PMC_IDX_FIXED) {
>> +	if (is_metric_idx(idx))
>> +		idx = 1 << 29;
> 
> I can't find this in the SDM RDPMC description. What does it return?

It will return the value of PERF_METRICS. I will add it in the changelog.

Thanks,
Kan
> 
>> +	else if (x86_pmu.num_counters_fixed && idx >= INTEL_PMC_IDX_FIXED) {
>>   		idx -= INTEL_PMC_IDX_FIXED;
>>   		idx |= 1 << 30;
>>   	}

  reply	other threads:[~2019-05-28 18:23 UTC|newest]

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-21 21:40 [PATCH 0/9] TopDown metrics support for Icelake kan.liang
2019-05-21 21:40 ` [PATCH 1/9] perf/core: Support a REMOVE transaction kan.liang
2019-05-21 21:40 ` [PATCH 2/9] perf/x86/intel: Basic support for metrics counters kan.liang
2019-05-28 12:05   ` Peter Zijlstra
2019-05-28 18:20     ` Liang, Kan
2019-05-28 12:15   ` Peter Zijlstra
2019-05-28 18:21     ` Liang, Kan
2019-05-29  7:28       ` Peter Zijlstra
2019-05-29 14:40         ` Liang, Kan
2019-05-29 16:46           ` Peter Zijlstra
2019-05-29  8:14   ` Peter Zijlstra
2019-05-21 21:40 ` [PATCH 3/9] perf/x86/intel: Support overflows on SLOTS kan.liang
2019-05-28 12:20   ` Peter Zijlstra
2019-05-28 18:22     ` Liang, Kan
2019-05-21 21:40 ` [PATCH 4/9] perf/x86/intel: Support hardware TopDown metrics kan.liang
2019-05-28 12:43   ` Peter Zijlstra
2019-05-28 18:23     ` Liang, Kan [this message]
2019-05-29  7:30       ` Peter Zijlstra
2019-05-28 12:53   ` Peter Zijlstra
2019-05-28 12:56   ` Peter Zijlstra
2019-05-28 13:32     ` Peter Zijlstra
2019-05-28 13:30   ` Peter Zijlstra
2019-05-28 18:24     ` Liang, Kan
2019-05-29  7:34       ` Peter Zijlstra
2019-05-29 14:41         ` Liang, Kan
2019-05-28 13:43   ` Peter Zijlstra
2019-05-28 18:24     ` Liang, Kan
2019-05-29  7:54       ` Peter Zijlstra
2019-05-29 14:42         ` Liang, Kan
2019-05-29 16:58           ` Peter Zijlstra
2019-06-04 20:39             ` Liang, Kan
2019-05-28 13:48   ` Peter Zijlstra
2019-05-28 18:24     ` Liang, Kan
2019-05-29  7:57       ` Peter Zijlstra
2019-05-29 14:42         ` Liang, Kan
2019-05-21 21:40 ` [PATCH 5/9] perf/x86/intel: Set correct weight for TopDown metrics events kan.liang
2019-05-28 13:50   ` Peter Zijlstra
2019-05-21 21:40 ` [PATCH 6/9] perf/x86/intel: Export new TopDown metrics events for Icelake kan.liang
2019-05-21 21:40 ` [PATCH 7/9] perf/x86/intel: Disable sampling read slots and topdown kan.liang
2019-05-28 13:52   ` Peter Zijlstra
2019-05-28 18:25     ` Liang, Kan
2019-05-29  7:58       ` Peter Zijlstra
2019-05-21 21:40 ` [PATCH 8/9] perf, tools, stat: Support new per thread TopDown metrics kan.liang
2019-05-21 21:40 ` [PATCH 9/9] perf, tools: Add documentation for topdown metrics kan.liang

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