perf: Fixes (#38935)

Release Notes:

- N/A

tooling/perf/src/lib.rs

@@ -261,8 +261,8 @@ impl Output {
                 // Only compare categories where both           meow
                 // runs have data.                              /
                 let mut other_data = other_categories.remove(&cat)?;
-                let mut max = 0.;
-                let mut min = 0.;
+                let mut max = f64::MIN;
+                let mut min = f64::MAX;
 
                 // Running totals for averaging out tests.
                 let mut r_total_numerator = 0.;
@@ -284,10 +284,15 @@ impl Output {
                     r_total_numerator += shift * f64::from(weight);
                     r_total_denominator += u32::from(weight);
                 }
-                let mean = r_total_numerator / f64::from(r_total_denominator);
-                // TODO: also aggregate standard deviation? That's harder to keep
-                // meaningful, though, since we dk which tests are correlated.
-                Some((cat, PerfDelta { max, mean, min }))
+                // There were no runs here!
+                if r_total_denominator == 0 {
+                    None
+                } else {
+                    let mean = r_total_numerator / f64::from(r_total_denominator);
+                    // TODO: also aggregate standard deviation? That's harder to keep
+                    // meaningful, though, since we dk which tests are correlated.
+                    Some((cat, PerfDelta { max, mean, min }))
+                }
             })
             .collect();
 

tooling/perf/src/main.rs

@@ -93,7 +93,10 @@ impl OutputKind<'_> {
         match self {
             OutputKind::Markdown => print!("{output}"),
             OutputKind::Json(ident) => {
-                let wspace_dir = std::env::var("CARGO_MANIFEST_DIR").unwrap();
+                // We're going to be in tooling/perf/$whatever.
+                let wspace_dir = PathBuf::from(std::env::var("CARGO_MANIFEST_DIR").unwrap())
+                    .join("..")
+                    .join("..");
                 let runs_dir = PathBuf::from(&wspace_dir).join(consts::RUNS_DIR);
                 std::fs::create_dir_all(&runs_dir).unwrap();
                 assert!(