Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown
 metrics
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
References: <20190826144740.10163-1-kan.liang@linux.intel.com>
 <20190826144740.10163-4-kan.liang@linux.intel.com>
 <20190828150238.GC17205@worktop.programming.kicks-ass.net>
 <20190828190445.GQ5447@tassilo.jf.intel.com>
 <20190831091931.GJ2369@hirez.programming.kicks-ass.net>
From:   "Liang, Kan" <kan.liang@linux.intel.com>
Message-ID: <9ec0a847-2b6b-3422-72bd-ac738f1c0d10@linux.intel.com>
Date:   Mon, 9 Sep 2019 09:40:26 -0400
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101
 Thunderbird/60.9.0
MIME-Version: 1.0
In-Reply-To: <20190831091931.GJ2369@hirez.programming.kicks-ass.net>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


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