perf(ecstore): converge disk metric atomic loads (#6866)

Use the seqlock version as the publication fence and keep payload reads relaxed while validating the final version. This reduces ordering overhead in disk metric recording and snapshot collection without changing the rolling-window contract.

Co-authored-by: heihutu <heihutu@gmail.com>

crates/ecstore/src/disk/disk_store.rs

@@ -678,17 +678,20 @@ impl DiskOperationMetrics {
         let elapsed_nanos = u64::try_from(elapsed.as_nanos()).unwrap_or(u64::MAX);
         let slot = &self.last_minute[(now_sec % 60) as usize];
         loop {
-            let version = slot.version.load(Ordering::Acquire);
+            // The successful CAS below is AcqRel, so it is the publication
+            // fence for the writer that owns this slot. The initial parity
+            // check does not need to acquire the slot payload.
+            let version = slot.version.load(Ordering::Relaxed);
             if !version.is_multiple_of(2) {
                 std::hint::spin_loop();
                 continue;
             }
             if slot
                 .version
-                .compare_exchange(version, version.wrapping_add(1), Ordering::AcqRel, Ordering::Acquire)
+                .compare_exchange(version, version.wrapping_add(1), Ordering::AcqRel, Ordering::Relaxed)
                 .is_ok()
             {
-                if slot.unix_sec.load(Ordering::Acquire) != now_sec {
+                if slot.unix_sec.load(Ordering::Relaxed) != now_sec {
                     slot.count.store(0, Ordering::Relaxed);
                     slot.acc_time.store(0, Ordering::Relaxed);
                     slot.unix_sec.store(now_sec, Ordering::Release);
@@ -704,14 +707,10 @@ impl DiskOperationMetrics {
     fn last_minute_snapshot(&self, now_sec: u64) -> TimedAction {
         let mut snapshot = TimedAction::default();
         for slot in &self.last_minute {
-            let version = slot.version.load(Ordering::Acquire);
-            if !version.is_multiple_of(2) {
+            let Some((slot_sec, count, acc_time)) = slot.snapshot() else {
                 continue;
-            }
-            let slot_sec = slot.unix_sec.load(Ordering::Acquire);
-            let count = slot.count.load(Ordering::Acquire);
-            let acc_time = slot.acc_time.load(Ordering::Acquire);
-            if slot.version.load(Ordering::Acquire) == version && slot_sec <= now_sec && now_sec.saturating_sub(slot_sec) < 60 {
+            };
+            if slot_sec <= now_sec && now_sec.saturating_sub(slot_sec) < 60 {
                 snapshot.count = snapshot.count.saturating_add(count);
                 snapshot.acc_time = snapshot.acc_time.saturating_add(acc_time);
             }
@@ -720,6 +719,23 @@ impl DiskOperationMetrics {
     }
 }
 
+impl TimedActionSlot {
+    fn snapshot(&self) -> Option<(u64, u64, u64)> {
+        let version = self.version.load(Ordering::Acquire);
+        if !version.is_multiple_of(2) {
+            return None;
+        }
+
+        // The first Acquire load publishes the payload written before the
+        // matching Release store. Relaxed payload loads are sufficient while
+        // the final Acquire version load validates that no writer intervened.
+        let slot_sec = self.unix_sec.load(Ordering::Relaxed);
+        let count = self.count.load(Ordering::Relaxed);
+        let acc_time = self.acc_time.load(Ordering::Relaxed);
+        (self.version.load(Ordering::Acquire) == version).then_some((slot_sec, count, acc_time))
+    }
+}
+
 pub(crate) struct DiskHealthWaitingGuard<'a> {
     health: &'a DiskHealthTracker,
 }