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From: Andi Kleen <ak@linux.intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: kan.liang@linux.intel.com, 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: Wed, 28 Aug 2019 20:11:51 -0700	[thread overview]
Message-ID: <20190829031151.GR5447@tassilo.jf.intel.com> (raw)
In-Reply-To: <20190828162857.GO2332@hirez.programming.kicks-ass.net>

On Wed, Aug 28, 2019 at 06:28:57PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 28, 2019 at 09:17:54AM -0700, Andi Kleen wrote:
> > > This really doesn't make sense to me; if you set FIXED_CTR3 := 0, you'll
> > > never trigger the overflow there; this then seems to suggest the actual
> > 
> > The 48bit counter might overflow in a few hours.
> 
> Sure; the point is? Kan said it should not be too big; a full 48bit wrap
> around must be too big or nothing is.

We expect users to avoid making it too big, but we cannot rule it out.

Actually for the common perf stat for a long time case you can make it fairly
big because the percentages still work out over the complete run time.

The problem with letting it accumulate too much is mainly if you
want to use a continuously running metrics counter to time smaller
regions by taking deltas, for example using RDPMC. 

In this case the small regions would be too inaccurate after some time.

But to make the first case work absolutely need to handle the overflow
case. Otherwise the metrics would just randomly stop at some
point.


-Andi




  reply	other threads:[~2019-08-29  3:11 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 [this message]
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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