From 98be918abd59b424c750b09d19b053718cbf1186 Mon Sep 17 00:00:00 2001
From: michaelfeil <me@michaelfeil.eu>
Date: Wed, 17 Dec 2025 19:38:15 +0000
Subject: [PATCH 1/3] candle: health check by queuing on cuda # What does this
 PR do?

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Fixes # (issue)

## Before submitting

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---
 backends/candle/src/lib.rs | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/backends/candle/src/lib.rs b/backends/candle/src/lib.rs
index ff824f555..739c494f1 100644
--- a/backends/candle/src/lib.rs
+++ b/backends/candle/src/lib.rs
@@ -581,6 +581,26 @@ impl Backend for CandleBackend {
     }
 
     fn health(&self) -> Result<(), BackendError> {
+        // Simple healthcheck by performing a trivial operation
+        // backend is almost unfailable, but e.g. Cuda OOM or cuda "device fallen off the bus"
+        // can be detected this way
+        use candle_core::Tensor;
+
+        // 1) enqueue a trivial op on the current device
+        let x = Tensor::new(&[1f32], &self.device).e()?;
+        let y = (&x * 2f32).e()?;
+
+        // 2) force completion + surface async CUDA errors by reading back
+        let v = y.to_vec1::<f32>().e()?;
+        if v.len() != 1 || (v[0] - 2.0).abs() > 1e-6 {
+            // ideally, we should sleep here for 1.0s to allow k8s to detect healthcheck failure
+            // without queuing further work on a possibly broken device by blocking the backend.
+            // and sending 429s in the meantime.
+            return Err(BackendError::Inference(format!(
+                "device healthcheck failed: expected [2.0], got {v:?}"
+            )));
+        }
+
         Ok(())
     }
 

From 28029afa757add6fc29b1cf88bcd1f40ab6f09c1 Mon Sep 17 00:00:00 2001
From: michaelfeil <me@michaelfeil.eu>
Date: Wed, 17 Dec 2025 19:45:50 +0000
Subject: [PATCH 2/3] add lib.rs

---
 backends/candle/src/lib.rs | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/backends/candle/src/lib.rs b/backends/candle/src/lib.rs
index 739c494f1..b3836afcd 100644
--- a/backends/candle/src/lib.rs
+++ b/backends/candle/src/lib.rs
@@ -584,16 +584,17 @@ impl Backend for CandleBackend {
         // Simple healthcheck by performing a trivial operation
         // backend is almost unfailable, but e.g. Cuda OOM or cuda "device fallen off the bus"
         // can be detected this way
-        use candle_core::Tensor;
+        use candle::Tensor;
 
         // 1) enqueue a trivial op on the current device
         let x = Tensor::new(&[1f32], &self.device).e()?;
-        let y = (&x * 2f32).e()?;
+        let z = Tensor::new(&[2f32], &self.device).e()?;
+        let y = (&x * &z).e()?;
 
-        // 2) force completion + surface async CUDA errors by reading back
+        // 2) move storage to CPU to surface async CUDA errors by reading back
         let v = y.to_vec1::<f32>().e()?;
         if v.len() != 1 || (v[0] - 2.0).abs() > 1e-6 {
-            // ideally, we should sleep here for 1.0s to allow k8s to detect healthcheck failure
+            // michaelfeil: ideally, we should sleep here for 1.0s/5.0s to allow k8s to detect healthcheck failure
             // without queuing further work on a possibly broken device by blocking the backend.
             // and sending 429s in the meantime.
             return Err(BackendError::Inference(format!(

From 22a0c871c603fb3b2e90528cd5cd4be4aec76050 Mon Sep 17 00:00:00 2001
From: michaelfeil <me@michaelfeil.eu>
Date: Wed, 17 Dec 2025 19:53:04 +0000
Subject: [PATCH 3/3] add logging

---
 backends/candle/src/lib.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/backends/candle/src/lib.rs b/backends/candle/src/lib.rs
index b3836afcd..5c3f41faa 100644
--- a/backends/candle/src/lib.rs
+++ b/backends/candle/src/lib.rs
@@ -597,10 +597,12 @@ impl Backend for CandleBackend {
             // michaelfeil: ideally, we should sleep here for 1.0s/5.0s to allow k8s to detect healthcheck failure
             // without queuing further work on a possibly broken device by blocking the backend.
             // and sending 429s in the meantime.
+            tracing::error!("Device {:?} healthcheck failed: expected [2.0], got {v:?}", self.device);
             return Err(BackendError::Inference(format!(
                 "device healthcheck failed: expected [2.0], got {v:?}"
             )));
         }
+        tracing::debug!("Device {:?} healthcheck passed", self.device);
 
         Ok(())
     }