fix: settle down to configured percent (#3947)

in real env testing we noted that the disk-usage based eviction sails 1
percentage point above the configured value, which might be a source of
confusion, so it might be better to get rid of that confusion now.

confusion: "I configured 85% but pageserver sails at 86%".

Co-authored-by: Christian Schwarz <christian@neon.tech>

libs/utils/src/serde_percent.rs

@@ -11,6 +11,14 @@ use serde::{Deserialize, Serialize};
 pub struct Percent(#[serde(deserialize_with = "deserialize_pct_0_to_100")] u8);
 
 impl Percent {
+    pub const fn new(pct: u8) -> Option<Self> {
+        if pct <= 100 {
+            Some(Percent(pct))
+        } else {
+            None
+        }
+    }
+
     pub fn get(&self) -> u8 {
         self.0
     }

pageserver/src/disk_usage_eviction_task.rs

@@ -639,7 +639,7 @@ mod filesystem_level_usage {
                 ),
                 (
                     "max_usage_pct",
-                    usage_pct > self.config.max_usage_pct.get() as u64,
+                    usage_pct >= self.config.max_usage_pct.get() as u64,
                 ),
             ];
 
@@ -686,4 +686,43 @@ mod filesystem_level_usage {
             avail_bytes,
         })
     }
+
+    #[test]
+    fn max_usage_pct_pressure() {
+        use super::Usage as _;
+        use std::time::Duration;
+        use utils::serde_percent::Percent;
+
+        let mut usage = Usage {
+            config: &DiskUsageEvictionTaskConfig {
+                max_usage_pct: Percent::new(85).unwrap(),
+                min_avail_bytes: 0,
+                period: Duration::MAX,
+                #[cfg(feature = "testing")]
+                mock_statvfs: None,
+            },
+            total_bytes: 100_000,
+            avail_bytes: 0,
+        };
+
+        assert!(usage.has_pressure(), "expected pressure at 100%");
+
+        usage.add_available_bytes(14_000);
+        assert!(usage.has_pressure(), "expected pressure at 86%");
+
+        usage.add_available_bytes(999);
+        assert!(usage.has_pressure(), "expected pressure at 85.001%");
+
+        usage.add_available_bytes(1);
+        assert!(usage.has_pressure(), "expected pressure at precisely 85%");
+
+        usage.add_available_bytes(1);
+        assert!(!usage.has_pressure(), "no pressure at 84.999%");
+
+        usage.add_available_bytes(999);
+        assert!(!usage.has_pressure(), "no pressure at 84%");
+
+        usage.add_available_bytes(16_000);
+        assert!(!usage.has_pressure());
+    }
 }