perf/x86/intel: Add INST_RETIRED.ALL workarounds

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@linux.intel.com>
[ Updated comments and changelog a bit. ]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1424225886-18652-3-git-send-email-andi@firstfloor.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index e0dab5c..ec6e982 100644 (file)
--- 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 *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 perf_event *event)
        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;
 
        /*

diff --git a/arch/x86/kernel/cpu/perf_event.h b/arch/x86/kernel/cpu/perf_event.h
index a371d27..87e5081 100644 (file)
--- 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

diff --git a/arch/x86/kernel/cpu/perf_event_intel.c b/arch/x86/kernel/cpu/perf_event_intel.c
index 2883853..fc6dbc4 100644 (file)
--- a/arch/x86/kernel/cpu/perf_event_intel.c
+++ b/arch/x86/kernel/cpu/perf_event_intel.c
@@ -2096,6 +2096,32 @@ hsw_get_event_constraints(struct cpu_hw_events *cpuc, struct perf_event *event)
        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"     );
@@ -2774,6 +2800,7 @@ __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;