[pbs-devel] [PATCH proxmox-backup v2 3/4] partial fix #6049: datastore: use config fast-path in Drop

[Samuel Rufinatscha]

The Drop impl of DataStore re-read datastore.cfg to decide whether
the entry should be evicted from the in-process cache (based on
maintenance mode’s clear_from_cache). During the investigation of
issue #6049 [1], a flamegraph [2] showed that the config reload in Drop
accounted for a measurable share of CPU time under load.

This patch adds the datastore config fast path to the Drop impl to
eventually avoid an expensive config reload from disk to capture
the maintenance mandate.

Links

[1] Bugzilla: https://bugzilla.proxmox.com/show_bug.cgi?id=6049
[2] cargo-flamegraph: https://github.com/flamegraph-rs/flamegraph

Fixes: #6049
Signed-off-by: Samuel Rufinatscha <s.rufinatscha at proxmox.com>
---
 pbs-datastore/src/datastore.rs | 43 +++++++++++++++++++++++++++-------
 1 file changed, 34 insertions(+), 9 deletions(-)

diff --git a/pbs-datastore/src/datastore.rs b/pbs-datastore/src/datastore.rs
index e7748872..0fabf592 100644
--- a/pbs-datastore/src/datastore.rs
+++ b/pbs-datastore/src/datastore.rs
@@ -214,15 +214,40 @@ impl Drop for DataStore {
             // remove datastore from cache iff
             //  - last task finished, and
             //  - datastore is in a maintenance mode that mandates it
-            let remove_from_cache = last_task
-                && pbs_config::datastore::config()
-                    .and_then(|(s, _)| s.lookup::<DataStoreConfig>("datastore", self.name()))
-                    .is_ok_and(|c| {
-                        c.get_maintenance_mode()
-                            .is_some_and(|m| m.clear_from_cache())
-                    });
-
-            if remove_from_cache {
+
+            // first check: check if last task finished
+            if !last_task {
+                return;
+            }
+
+            let (section_config, _gen) = match datastore_section_config_cached() {
+                Ok(v) => v,
+                Err(err) => {
+                    log::error!(
+                        "failed to load datastore config in Drop for {} - {err}",
+                        self.name()
+                    );
+                    return;
+                }
+            };
+
+            let datastore_cfg: DataStoreConfig =
+                match section_config.lookup("datastore", self.name()) {
+                    Ok(cfg) => cfg,
+                    Err(err) => {
+                        log::error!(
+                            "failed to look up datastore '{}' in Drop - {err}",
+                            self.name()
+                        );
+                        return;
+                    }
+                };
+
+            // second check: check maintenance mode mandate
+            if datastore_cfg
+                .get_maintenance_mode()
+                .is_some_and(|m| m.clear_from_cache())
+            {
                 DATASTORE_MAP.lock().unwrap().remove(self.name());
             }
         }
-- 
2.47.3