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Date: Mon, 23 Mar 2015 13:35:07 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 3/3] perf, x86: Add INST_RETIRED.ALL workarounds
On Mon, Mar 23, 2015 at 11:39:00AM +0100, Ingo Molnar wrote:
> > http://www.intel.com/content/dam/www/public/us/en/documents/specification-updates/5th-gen-core-family-spec-update.pdf
> >
> > BDM11 and BDM55 (not 57) tell us that the PMU will generate crap output
> > if you don't do this. Non-fatal but gibberish.
>
> Should be part of the changelog?
Sure, lemme go make that happen.
> So I did not say rounding up, I meant this sentence:
>
> > > > + * [...] We combine the two to enforce
> > > > + * a min-period of 128.
>
> IMO ambiguously suggests that the result of the combination of the two
> is to enforce a min-period of 128. Would somethin like this:
>
> We combine the two to enforce
> a min-period of 128, rounded (down) to multiples of 64.
> The original period is still kept by the core code and is
> approximated in the long run via these slightly fuzzed
> hardware-periods.
Like so then?
---
Subject: perf, x86: Add INST_RETIRED.ALL workarounds
From: Andi Kleen <ak@...ux.intel.com>
Date: Tue, 17 Feb 2015 18:18:06 -0800
On Broadwell INST_RETIRED.ALL cannot be used with any period
that doesn't have the lowest 6 bits cleared. And the period
should not be smaller than 128.
This is erratum BDM11 and BDM55;
http://www.intel.com/content/dam/www/public/us/en/documents/specification-updates/5th-gen-core-family-spec-update.pdf
BDM11: When using a period < 100; we may get incorrect PEBS/PMI
interrupts and/or an invalid counter state.
BDM55: When bit0-5 of the period are !0 we may get redundant PEBS
records on overflow.
Add a new callback to enforce this, and set it for Broadwell.
How does this handle the case when an app requests a specific
period with some of the bottom bits set?
Short answer:
Any useful instruction sampling period needs to be 4-6 orders
of magnitude larger than 128, as an PMI every 128 instructions
would instantly overwhelm the system and be throttled.
So the +-64 error from this is really small compared to the
period, much smaller than normal system jitter.
Long answer (by Peterz):
IFF we guarantee perf_event_attr::sample_period >= 128.
Suppose we start out with sample_period=192; then we'll set period_left
to 192, we'll end up with left = 128 (we truncate the lower bits). We
get an interrupt, find that period_left = 64 (>0 so we return 0 and
don't get an overflow handler), up that to 128. Then we trigger again,
at n=256. Then we find period_left = -64 (<=0 so we return 1 and do get
an overflow). We increment with sample_period so we get left = 128. We
fire again, at n=384, period_left = 0 (<=0 so we return 1 and get an
overflow). And on and on.
So while the individual interrupts are 'wrong' we get then with
interval=256,128 in exactly the right ratio to average out at 192. And
this works for everything >=128.
So the num_samples*fixed_period thing is still entirely correct +- 127,
which is good enough I'd say, as you already have that error anyhow.
So no need to 'fix' the tools, al we need to do is refuse to create
INST_RETIRED:ALL events with sample_period < 128.
Signed-off-by: Andi Kleen <ak@...ux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
arch/x86/kernel/cpu/perf_event.c | 9 +++++++++
arch/x86/kernel/cpu/perf_event.h | 1 +
arch/x86/kernel/cpu/perf_event_intel.c | 27 +++++++++++++++++++++++++++
3 files changed, 37 insertions(+)
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -451,6 +451,12 @@ int x86_pmu_hw_config(struct perf_event
if (event->attr.type == PERF_TYPE_RAW)
event->hw.config |= event->attr.config & X86_RAW_EVENT_MASK;
+ if (event->attr.sample_period && x86_pmu.limit_period) {
+ if (x86_pmu.limit_period(event, event->attr.sample_period) >
+ event->attr.sample_period)
+ return -EINVAL;
+ }
+
return x86_setup_perfctr(event);
}
@@ -988,6 +994,9 @@ int x86_perf_event_set_period(struct per
if (left > x86_pmu.max_period)
left = x86_pmu.max_period;
+ if (x86_pmu.limit_period)
+ left = x86_pmu.limit_period(event, left);
+
per_cpu(pmc_prev_left[idx], smp_processor_id()) = left;
/*
--- a/arch/x86/kernel/cpu/perf_event.h
+++ b/arch/x86/kernel/cpu/perf_event.h
@@ -451,6 +451,7 @@ struct x86_pmu {
struct x86_pmu_quirk *quirks;
int perfctr_second_write;
bool late_ack;
+ unsigned (*limit_period)(struct perf_event *event, unsigned l);
/*
* sysfs attrs
--- a/arch/x86/kernel/cpu/perf_event_intel.c
+++ b/arch/x86/kernel/cpu/perf_event_intel.c
@@ -2097,6 +2097,32 @@ hsw_get_event_constraints(struct cpu_hw_
return c;
}
+/*
+ * Broadwell:
+ *
+ * The INST_RETIRED.ALL period always needs to have lowest 6 bits cleared
+ * (BDM55) and it must not use a period smaller than 100 (BDM11). We combine
+ * the two to enforce a minimum period of 128 (the smallest value that has bits
+ * 0-5 cleared and >= 100).
+ *
+ * Because of how the code in x86_perf_event_set_period() works, the truncation
+ * of the lower 6 bits is 'harmless' as we'll occasionally add a longer period
+ * to make up for the 'lost' events due to carrying the 'error' in period_left.
+ *
+ * Therefore the effective (average) period matches the requested period,
+ * despite coarser hardware granularity.
+ */
+static unsigned bdw_limit_period(struct perf_event *event, unsigned left)
+{
+ if ((event->hw.config & INTEL_ARCH_EVENT_MASK) ==
+ X86_CONFIG(.event=0xc0, .umask=0x01)) {
+ if (left < 128)
+ left = 128;
+ left &= ~0x3fu;
+ }
+ return left;
+}
+
PMU_FORMAT_ATTR(event, "config:0-7" );
PMU_FORMAT_ATTR(umask, "config:8-15" );
PMU_FORMAT_ATTR(edge, "config:18" );
@@ -2775,6 +2801,7 @@ __init int intel_pmu_init(void)
x86_pmu.hw_config = hsw_hw_config;
x86_pmu.get_event_constraints = hsw_get_event_constraints;
x86_pmu.cpu_events = hsw_events_attrs;
+ x86_pmu.limit_period = bdw_limit_period;
pr_cont("Broadwell events, ");
break;
--
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