From: Peter Zijlstra <peterz@infradead.org>
To: Stephane Eranian <eranian@google.com>
Cc: Andi Kleen <ak@linux.intel.com>,
"Liang, Kan" <kan.liang@linux.intel.com>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
Ingo Molnar <mingo@redhat.com>,
LKML <linux-kernel@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Jiri Olsa <jolsa@kernel.org>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>
Subject: Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics
Date: Sat, 31 Aug 2019 11:29:21 +0200 [thread overview]
Message-ID: <20190831092921.GK2369@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CABPqkBSPfZ6iQ_t1ESZf334q+dNVf=RvoNHdmKZSs5GLokbjFQ@mail.gmail.com>
On Sat, Aug 31, 2019 at 02:13:05AM -0700, Stephane Eranian wrote:
> With PERF_METRICS, the delta is always since previous read. If you read
> frequently enough you do not lose precision.
You always loose precision, the whole fraction of 255 looses the
fractional part; consider:
255 * 1 / 8
31.87500000000000000000
255 * 7 / 8
223.12500000000000000000
31 * 8 / 255
.97254901960784313725
223 * 8 / 255
6.99607843137254901960
Now, we can make reconstruction use normal 'round-nearest' and then we'd
get 1 and 7 back, but there's always cases where you loose things.
Also, with 255 being an odd number, round-nearest is actually 'hard' :/
next prev parent reply other threads:[~2019-08-31 9:29 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
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 [this message]
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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