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From: Peter Zijlstra <peterz@infradead.org>
To: "Liang, Kan" <kan.liang@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, ak@linux.intel.com
Subject: Re: [PATCH 4/9] perf/x86/intel: Support hardware TopDown metrics
Date: Wed, 29 May 2019 09:34:21 +0200	[thread overview]
Message-ID: <20190529073421.GZ2623@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <a3722bae-9506-21f0-7e6e-a85217313bf8@linux.intel.com>

On Tue, May 28, 2019 at 02:24:14PM -0400, Liang, Kan wrote:

> > > +		if (!(txn_flags & PERF_PMU_TXN_REMOVE)) {
> > > +			/* Reset the metric value when reading
> > > +			 * The SLOTS register must be reset when PERF_METRICS reset,
> > > +			 * otherwise PERF_METRICS may has wrong output.
> > > +			 */
> > 
> > broken comment style.. (and grammer)
> 
> Missed a full stop.
> Should be "Reset the metric value for each read."

s/may has wrong/may have wrong/

> > > +			wrmsrl(MSR_PERF_METRICS, 0);
> > > +			wrmsrl(MSR_CORE_PERF_FIXED_CTR3, 0);
> > 
> > I don't get this, overflow happens on when we flip sign, so why is
> > programming 0 a sane thing to do?
> 
> Reset the counters (programming 0) don't trigger overflow.

Right, so why then do you allow creating this thing as
is_sampling_event() ?

> We have to reset both registers for each read to avoid the known
> PERF_METRICS issue.

'the known PERF_METRICS issue' is unknown to me and any other reader.

> > > +	metric = (cpuc->last_metric >> ((hwc->idx - INTEL_PMC_IDX_FIXED_METRIC_BASE)*8)) & 0xff;
> > > +	last = (((metric * 0xffff) >> 8) * cpuc->last_slots) >> 16;
> > 
> > How is that cpuc->last_* crap not broken for NMIs ?
> 
> There should be no NMI for slots or metric events at the moment, because the
> MSR_PERF_METRICS and MSR_CORE_PERF_FIXED_CTR3 are reset in first read.
> Other NMIs will not touch the codes here.

What happens if someone does: read(perf_fd) and then has the NMI hit?

  reply	other threads:[~2019-05-29  7:34 UTC|newest]

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-21 21:40 [PATCH 0/9] TopDown metrics support for Icelake kan.liang
2019-05-21 21:40 ` [PATCH 1/9] perf/core: Support a REMOVE transaction kan.liang
2019-05-21 21:40 ` [PATCH 2/9] perf/x86/intel: Basic support for metrics counters kan.liang
2019-05-28 12:05   ` Peter Zijlstra
2019-05-28 18:20     ` Liang, Kan
2019-05-28 12:15   ` Peter Zijlstra
2019-05-28 18:21     ` Liang, Kan
2019-05-29  7:28       ` Peter Zijlstra
2019-05-29 14:40         ` Liang, Kan
2019-05-29 16:46           ` Peter Zijlstra
2019-05-29  8:14   ` Peter Zijlstra
2019-05-21 21:40 ` [PATCH 3/9] perf/x86/intel: Support overflows on SLOTS kan.liang
2019-05-28 12:20   ` Peter Zijlstra
2019-05-28 18:22     ` Liang, Kan
2019-05-21 21:40 ` [PATCH 4/9] perf/x86/intel: Support hardware TopDown metrics kan.liang
2019-05-28 12:43   ` Peter Zijlstra
2019-05-28 18:23     ` Liang, Kan
2019-05-29  7:30       ` Peter Zijlstra
2019-05-28 12:53   ` Peter Zijlstra
2019-05-28 12:56   ` Peter Zijlstra
2019-05-28 13:32     ` Peter Zijlstra
2019-05-28 13:30   ` Peter Zijlstra
2019-05-28 18:24     ` Liang, Kan
2019-05-29  7:34       ` Peter Zijlstra [this message]
2019-05-29 14:41         ` Liang, Kan
2019-05-28 13:43   ` Peter Zijlstra
2019-05-28 18:24     ` Liang, Kan
2019-05-29  7:54       ` Peter Zijlstra
2019-05-29 14:42         ` Liang, Kan
2019-05-29 16:58           ` Peter Zijlstra
2019-06-04 20:39             ` Liang, Kan
2019-05-28 13:48   ` Peter Zijlstra
2019-05-28 18:24     ` Liang, Kan
2019-05-29  7:57       ` Peter Zijlstra
2019-05-29 14:42         ` Liang, Kan
2019-05-21 21:40 ` [PATCH 5/9] perf/x86/intel: Set correct weight for TopDown metrics events kan.liang
2019-05-28 13:50   ` Peter Zijlstra
2019-05-21 21:40 ` [PATCH 6/9] perf/x86/intel: Export new TopDown metrics events for Icelake kan.liang
2019-05-21 21:40 ` [PATCH 7/9] perf/x86/intel: Disable sampling read slots and topdown kan.liang
2019-05-28 13:52   ` Peter Zijlstra
2019-05-28 18:25     ` Liang, Kan
2019-05-29  7:58       ` Peter Zijlstra
2019-05-21 21:40 ` [PATCH 8/9] perf, tools, stat: Support new per thread TopDown metrics kan.liang
2019-05-21 21:40 ` [PATCH 9/9] perf, tools: Add documentation for topdown metrics kan.liang

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