All of lore.kernel.org
 help / color / mirror / Atom feed
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' :/

  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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20190831092921.GK2369@hirez.programming.kicks-ass.net \
    --to=peterz@infradead.org \
    --cc=acme@kernel.org \
    --cc=ak@linux.intel.com \
    --cc=alexander.shishkin@linux.intel.com \
    --cc=eranian@google.com \
    --cc=jolsa@kernel.org \
    --cc=kan.liang@linux.intel.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@redhat.com \
    --cc=tglx@linutronix.de \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.