From: "Liang, Kan" <kan.liang@linux.intel.com>
To: Peter Zijlstra <peterz@infradead.org>
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: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics
Date: Thu, 29 Aug 2019 09:31:37 -0400 [thread overview]
Message-ID: <fd95a255-4499-2907-8af9-d340f157da68@linux.intel.com> (raw)
In-Reply-To: <20190828151921.GD17205@worktop.programming.kicks-ass.net>
On 8/28/2019 11:19 AM, Peter Zijlstra wrote:
>> +static int icl_set_topdown_event_period(struct perf_event *event)
>> +{
>> + struct hw_perf_event *hwc = &event->hw;
>> + s64 left = local64_read(&hwc->period_left);
>> +
>> + /*
>> + * Clear PERF_METRICS and Fixed counter 3 in initialization.
>> + * After that, both MSRs will be cleared for each read.
>> + * Don't need to clear them again.
>> + */
>> + if (left == x86_pmu.max_period) {
>> + wrmsrl(MSR_CORE_PERF_FIXED_CTR3, 0);
>> + wrmsrl(MSR_PERF_METRICS, 0);
>> + local64_set(&hwc->period_left, 0);
>> + }
> 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
> counter value is irrelevant. Therefore you don't actually need this.
>
Could you please elaborate on why initialization to 0 never triggers an
overflow?
As of my understanding, initialization to 0 only means that it will take
more time than initialization to -max_period (0x8000 0000 0001) to
trigger an overflow.
Maybe 0 is too tricky. We can set the initial value to 1.
I think the bottom line is that we need a small initial value for
FIXED_CTR3 here.
PERF_METRICS reports an 8bit integer fraction which is something like
0xff * internal counters / FIXCTR3.
The internal counters only start counting from 0. (SW cannot set an
arbitrary initial value for internal counters.)
If the initial value of FIXED_CTR3 is too big, PERF_METRICS could always
remain constant, e.g. 0.
Thanks,
Kan
next prev parent reply other threads:[~2019-08-29 13:31 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 [this message]
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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