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From: "Liang, Kan" <kan.liang@linux.intel.com>
To: Peter Zijlstra <peterz@infradead.org>, Andi Kleen <ak@linux.intel.com>
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
Subject: Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics
Date: Mon, 9 Sep 2019 09:40:26 -0400	[thread overview]
Message-ID: <9ec0a847-2b6b-3422-72bd-ac738f1c0d10@linux.intel.com> (raw)
In-Reply-To: <20190831091931.GJ2369@hirez.programming.kicks-ass.net>



On 8/31/2019 5:19 AM, Peter Zijlstra wrote:
>>> Then there is no mucking about with that odd counter/metrics msr pair
>>> reset nonsense. Becuase that really stinks.
>> You have to write them to reset the internal counters.
> But not for ever read, only on METRIC_OVF.

The precision are lost if the counters were running much longer than the 
measured delta. We have to reset the counters after every read.
For example, the worst case is:
The previous reading was rounded up and the current reading is rounded 
down. The error of METRICS is 1/256 - 1/SLOTS, which is related to 
SLOTS. The error for each Topdown event is  (1/256 - 1/SLOTS) * SLOTS.
With the SLOTS increasing, the precision will get worse and worse.
Therefor, we have to reset the counters periodically.

Thanks,
Kan



  reply	other threads:[~2019-09-09 13:40 UTC|newest]

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-26 14:47 [RESEND PATCH V3 0/8] TopDown metrics support for Icelake kan.liang
2019-08-26 14:47 ` [RESEND PATCH V3 1/8] perf/x86/intel: Set correct mask for TOPDOWN.SLOTS kan.liang
2019-08-28  7:48   ` Peter Zijlstra
2019-08-26 14:47 ` [RESEND PATCH V3 2/8] perf/x86/intel: Basic support for metrics counters kan.liang
2019-08-28  7:48   ` Peter Zijlstra
2019-08-28  7:52   ` Peter Zijlstra
2019-08-28 13:59     ` Liang, Kan
2019-08-28  8:44   ` Peter Zijlstra
2019-08-28  9:02     ` Peter Zijlstra
2019-08-28  9:37       ` Peter Zijlstra
2019-08-28 13:51       ` Liang, Kan
2019-08-28  8:52   ` Peter Zijlstra
2019-08-26 14:47 ` [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics kan.liang
2019-08-28 15:02   ` Peter Zijlstra
2019-08-28 19:04     ` Andi Kleen
2019-08-31  9:19       ` Peter Zijlstra
2019-09-09 13:40         ` Liang, Kan [this message]
2019-08-28 19:35     ` Liang, Kan
2019-08-28 15:19   ` Peter Zijlstra
2019-08-28 16:11     ` [PATCH] x86/math64: Provide a sane mul_u64_u32_div() implementation for x86_64 Peter Zijlstra
2019-08-29  9:30       ` Peter Zijlstra
2019-08-28 16:17     ` [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics Andi Kleen
2019-08-28 16:28       ` Peter Zijlstra
2019-08-29  3:11         ` Andi Kleen
2019-08-29  9:17           ` Peter Zijlstra
2019-08-29 13:31     ` Liang, Kan
2019-08-29 13:52       ` Peter Zijlstra
2019-08-29 16:56         ` Liang, Kan
2019-08-31  9:18           ` Peter Zijlstra
2019-08-30 23:18   ` Stephane Eranian
2019-08-31  0:31     ` Andi Kleen
2019-08-31  9:13       ` Stephane Eranian
2019-08-31  9:29         ` Peter Zijlstra
2019-08-31 17:53         ` Andi Kleen
2019-08-26 14:47 ` [RESEND PATCH V3 4/8] perf/x86/intel: Support per thread RDPMC " kan.liang
2019-08-26 14:47 ` [RESEND PATCH V3 5/8] perf/x86/intel: Export TopDown events for Icelake kan.liang
2019-08-26 14:47 ` [RESEND PATCH V3 6/8] perf/x86/intel: Disable sampling read slots and topdown kan.liang
2019-08-26 14:47 ` [RESEND PATCH V3 7/8] perf, tools, stat: Support new per thread TopDown metrics kan.liang
2019-08-26 14:47 ` [RESEND PATCH V3 8/8] perf, tools: Add documentation for topdown metrics kan.liang

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