metrics: Expose process_cpu_seconds_total as a float (#754)

Prometheus handles all values as `f64`, but we only expose values as
whole integers. This means that the `process_cpu_seconds_total` metric
only exposes whole second values, while Linux exposes process time in
10ms increments.

This change modifies the `Counter` metric type to store an additional
marker that provides a strategy for converting the stored `u64` value to
`f64` for export.  This strategy is employed so that we can continue to
use `AtomicU64` to back counters and only use floats at export-time.  By
default the unit type is used to convert counters as before, but an
alternate `MillisAsSeconds` strategy is used to expose fractional
seconds from a millisecond counter.

This necessitates changing the histogram buckets to floats as well.
While this change doesn't modify the bucket values, this sets up future
changes to latency metrics.

linkerd/app/core/src/telemetry/process.rs

@@ -54,14 +54,14 @@ impl FmtMetrics for Report {
 #[cfg(target_os = "linux")]
 mod system {
     use libc::{self, pid_t};
-    use linkerd2_metrics::{metrics, Counter, FmtMetrics, Gauge};
+    use linkerd2_metrics::{metrics, Counter, FmtMetrics, Gauge, MillisAsSeconds};
     use procinfo::pid;
     use std::fmt;
     use std::{fs, io};
     use tracing::{error, warn};
 
     metrics! {
-        process_cpu_seconds_total: Counter {
+        process_cpu_seconds_total: Counter<MillisAsSeconds> {
             "Total user and system CPU time spent in seconds."
         },
         process_open_fds: Gauge { "Number of open file descriptors." },
@@ -77,16 +77,28 @@ mod system {
     #[derive(Clone, Debug)]
     pub(super) struct System {
         page_size: u64,
-        clock_ticks_per_sec: u64,
+        ms_per_tick: u64,
     }
 
     impl System {
         pub fn new() -> io::Result<Self> {
             let page_size = Self::sysconf(libc::_SC_PAGESIZE, "page size")?;
+
+            // On Linux, CLK_TCK is ~always `100`, so pure integer division
+            // works. This is probably not suitable if we encounter other
+            // values.
             let clock_ticks_per_sec = Self::sysconf(libc::_SC_CLK_TCK, "clock ticks per second")?;
+            let ms_per_tick = 1_000 / clock_ticks_per_sec;
+            if clock_ticks_per_sec != 100 {
+                warn!(
+                    clock_ticks_per_sec,
+                    ms_per_tick, "Unexpected value; process_cpu_seconds_total may be inaccurate."
+                );
+            }
+
             Ok(Self {
                 page_size,
-                clock_ticks_per_sec,
+                ms_per_tick,
             })
         }
 
@@ -130,9 +142,16 @@ mod system {
             };
 
             let clock_ticks = stat.utime as u64 + stat.stime as u64;
-            let cpu = Counter::from(clock_ticks / self.clock_ticks_per_sec);
+            let cpu_ms = clock_ticks * self.ms_per_tick;
             process_cpu_seconds_total.fmt_help(f)?;
-            process_cpu_seconds_total.fmt_metric(f, &cpu)?;
+            process_cpu_seconds_total.fmt_metric(f, &Counter::from(cpu_ms))?;
+
+            process_virtual_memory_bytes.fmt_help(f)?;
+            process_virtual_memory_bytes.fmt_metric(f, &Gauge::from(stat.vsize as u64))?;
+
+            process_resident_memory_bytes.fmt_help(f)?;
+            process_resident_memory_bytes
+                .fmt_metric(f, &Gauge::from(stat.rss as u64 * self.page_size))?;
 
             match Self::open_fds(stat.pid) {
                 Ok(open_fds) => {
@@ -141,7 +160,6 @@ mod system {
                 }
                 Err(err) => {
                     warn!("could not determine process_open_fds: {}", err);
-                    return Ok(());
                 }
             }
 
@@ -153,17 +171,10 @@ mod system {
                 }
                 Err(err) => {
                     warn!("could not determine process_max_fds: {}", err);
-                    return Ok(());
                 }
             }
 
-            process_virtual_memory_bytes.fmt_help(f)?;
-            let vsz = Gauge::from(stat.vsize as u64);
-            process_virtual_memory_bytes.fmt_metric(f, &vsz)?;
-
-            process_resident_memory_bytes.fmt_help(f)?;
-            let rss = Gauge::from(stat.rss as u64 * self.page_size);
-            process_resident_memory_bytes.fmt_metric(f, &rss)
+            Ok(())
         }
     }
 }

linkerd/metrics/src/counter.rs

@@ -1,4 +1,7 @@
-use super::prom::{FmtLabels, FmtMetric, MAX_PRECISE_VALUE};
+use super::{
+    prom::{FmtLabels, FmtMetric},
+    Factor,
+};
 use std::fmt::{self, Display};
 use std::sync::atomic::{AtomicU64, Ordering};
 
@@ -14,41 +17,58 @@ use std::sync::atomic::{AtomicU64, Ordering};
 /// [`rate()`]: https://prometheus.io/docs/prometheus/latest/querying/functions/#rate()
 /// [`irate()`]: https://prometheus.io/docs/prometheus/latest/querying/functions/#irate()
 /// [`resets()`]: https://prometheus.io/docs/prometheus/latest/querying/functions/#resets
-#[derive(Debug, Default)]
-pub struct Counter(AtomicU64);
+#[derive(Debug)]
+pub struct Counter<F = ()>(AtomicU64, std::marker::PhantomData<F>);
 
 // ===== impl Counter =====
 
-impl Counter {
+impl<F> Default for Counter<F> {
+    fn default() -> Self {
+        Self(AtomicU64::default(), std::marker::PhantomData)
+    }
+}
+
+impl<F> Counter<F> {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
     pub fn incr(&self) {
         self.add(1)
     }
 
     pub fn add(&self, n: u64) {
         self.0.fetch_add(n, Ordering::Release);
     }
+}
 
+impl<F: Factor> Counter<F> {
     /// Return current counter value, wrapped to be safe for use with Prometheus.
-    pub fn value(&self) -> u64 {
-        self.0
-            .load(Ordering::Acquire)
-            .wrapping_rem(MAX_PRECISE_VALUE + 1)
+    pub fn value(&self) -> f64 {
+        let n = self.0.load(Ordering::Acquire);
+        F::factor(n)
     }
 }
 
-impl Into<u64> for Counter {
-    fn into(self) -> u64 {
+impl<F: Factor> Into<f64> for &Counter<F> {
+    fn into(self) -> f64 {
         self.value()
     }
 }
 
-impl From<u64> for Counter {
+impl<F> Into<u64> for &Counter<F> {
+    fn into(self) -> u64 {
+        self.0.load(Ordering::Acquire)
+    }
+}
+
+impl<F> From<u64> for Counter<F> {
     fn from(value: u64) -> Self {
-        Counter(value.into())
+        Counter(value.into(), std::marker::PhantomData)
     }
 }
 
-impl FmtMetric for Counter {
+impl<F: Factor> FmtMetric for Counter<F> {
     const KIND: &'static str = "counter";
 
     fn fmt_metric<N: Display>(&self, f: &mut fmt::Formatter<'_>, name: N) -> fmt::Result {
@@ -74,34 +94,50 @@ impl FmtMetric for Counter {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::{MillisAsSeconds, MAX_PRECISE_UINT64};
 
     #[test]
     fn count_simple() {
-        let cnt = Counter::from(0);
-        assert_eq!(cnt.value(), 0);
-        cnt.incr();
-        assert_eq!(cnt.value(), 1);
-        cnt.add(41);
-        assert_eq!(cnt.value(), 42);
-        cnt.add(0);
-        assert_eq!(cnt.value(), 42);
+        let c = Counter::<()>::default();
+        assert_eq!(c.value(), 0.0);
+        c.incr();
+        assert_eq!(c.value(), 1.0);
+        c.add(41);
+        assert_eq!(c.value(), 42.0);
+        c.add(0);
+        assert_eq!(c.value(), 42.0);
     }
 
     #[test]
     fn count_wrapping() {
-        let cnt = Counter::from(MAX_PRECISE_VALUE - 1);
-        assert_eq!(cnt.value(), MAX_PRECISE_VALUE - 1);
-        cnt.incr();
-        assert_eq!(cnt.value(), MAX_PRECISE_VALUE);
-        cnt.incr();
-        assert_eq!(cnt.value(), 0);
-        cnt.incr();
-        assert_eq!(cnt.value(), 1);
-
-        let max = Counter::from(MAX_PRECISE_VALUE);
-        assert_eq!(max.value(), MAX_PRECISE_VALUE);
-
-        let over = Counter::from(MAX_PRECISE_VALUE + 1);
-        assert_eq!(over.value(), 0);
+        let c = Counter::<()>::from(MAX_PRECISE_UINT64 - 1);
+        assert_eq!(c.value(), (MAX_PRECISE_UINT64 - 1) as f64);
+        c.incr();
+        assert_eq!(c.value(), MAX_PRECISE_UINT64 as f64);
+        c.incr();
+        assert_eq!(c.value(), 0.0);
+        c.incr();
+        assert_eq!(c.value(), 1.0);
+
+        let max = Counter::<()>::from(MAX_PRECISE_UINT64);
+        assert_eq!(max.value(), MAX_PRECISE_UINT64 as f64);
+    }
+
+    #[test]
+    fn millis_as_seconds() {
+        let c = Counter::<MillisAsSeconds>::from(1);
+        assert_eq!(c.value(), 0.001);
+
+        let c = Counter::<MillisAsSeconds>::from((MAX_PRECISE_UINT64 - 1) * 1000);
+        assert_eq!(c.value(), (MAX_PRECISE_UINT64 - 1) as f64);
+        c.add(1000);
+        assert_eq!(c.value(), MAX_PRECISE_UINT64 as f64);
+        c.add(1000);
+        assert_eq!(c.value(), 0.0);
+        c.add(1000);
+        assert_eq!(c.value(), 1.0);
+
+        let max = Counter::<MillisAsSeconds>::from(MAX_PRECISE_UINT64 * 1000);
+        assert_eq!(max.value(), MAX_PRECISE_UINT64 as f64);
     }
 }

linkerd/metrics/src/gauge.rs

@@ -1,4 +1,4 @@
-use super::prom::{FmtLabels, FmtMetric, MAX_PRECISE_VALUE};
+use super::prom::{FmtLabels, FmtMetric};
 use std::fmt::{self, Display};
 use std::sync::atomic::{AtomicU64, Ordering};
 
@@ -20,7 +20,7 @@ impl Gauge {
     pub fn value(&self) -> u64 {
         self.0
             .load(Ordering::Acquire)
-            .wrapping_rem(MAX_PRECISE_VALUE + 1)
+            .wrapping_rem(crate::MAX_PRECISE_UINT64 + 1)
     }
 }