Add task_clock as additional counter used with perf counters

Currently all of sightglass's measuring units are tied processor events
which is only useful in comparing performance where the cpu remains
constant. This patch adds cpu time software counter as calculated by
the kernel.

crates/recorder/src/measure/counters.rs

@@ -4,7 +4,7 @@
 //! kernel.perf_event_paranoid=0`.
 use super::Measure;
 use crate::measure::Measurements;
-use perf_event::{events::Hardware, Builder, Counter, Group};
+use perf_event::{events::{Hardware, Software}, Builder, Counter, Group};
 use serde::{Deserialize, Serialize};
 use sightglass_data::Phase;
 
@@ -15,6 +15,7 @@ pub struct CounterMeasure {
     instructions_retired: Counter,
     cache_accesses: Counter,
     cache_misses: Counter,
+    task_clock: Counter,
 }
 
 impl CounterMeasure {
@@ -60,6 +61,15 @@ impl CounterMeasure {
                      have such a counter? If it does, your kernel may not fully support this \
                      processor.",
                 ),
+            task_clock: Builder::new()
+                .group(&mut group)
+                .kind(Software::TASK_CLOCK)
+                .build()
+                .expect(
+                    "Unable to create TASK_CLOCK software counter. Does this system actually \
+                     have such a counter? If it does, your kernel may not fully support this \
+                     processor.",
+                ),
             event_group: group,
         }
     }
@@ -83,6 +93,7 @@ impl Measure for CounterMeasure {
         );
         measurements.add(phase, "cache-accesses".into(), counts[&self.cache_accesses]);
         measurements.add(phase, "cache-misses".into(), counts[&self.cache_misses]);
+        measurements.add(phase, "task-clocks".into(), counts[&self.task_clock]);
     }
 }
 
@@ -102,6 +113,9 @@ pub struct PerfCounters {
     /// Measured by performance counter. May be 0, in which case the counter is almost certainly
     /// disabled.
     pub cache_misses: u64,
+    /// Measured by performance counter. May be 0, in which case the counter is almost certainly
+    /// disabled.
+    pub task_clock: u64,
 }
 
 impl std::ops::Div<u64> for PerfCounters {
@@ -112,6 +126,7 @@ impl std::ops::Div<u64> for PerfCounters {
             instructions_retired: self.instructions_retired / rhs,
             cache_accesses: self.cache_accesses / rhs,
             cache_misses: self.cache_misses / rhs,
+            task_clock: self.task_clock / rhs,
         }
     }
 }
@@ -124,6 +139,7 @@ impl std::ops::Add<PerfCounters> for PerfCounters {
             instructions_retired: self.instructions_retired + rhs.instructions_retired,
             cache_accesses: self.cache_accesses + rhs.cache_accesses,
             cache_misses: self.cache_misses + rhs.cache_misses,
+            task_clock: self.task_clock + rhs.task_clock,
         }
     }
 }