[perf] Check caching on IncRef'd dentries before the others.

When a child is added to a parent (directory) dentry, both child and parent are
queued for checkCachingLocked(). Make sure that the parent is queued first
because the parent gained a ref and so could be removed from the LRU cache hence
making space for the new child. This could prevent an LRU cache eviction.

In practice, this did seem to help. ~800 RPCs were reduced while building
//absl/... (ABSL build benchmark). Evictions hurt in 2 ways - create renameMu
contention and destroy a possibly useful dentry which will have to be re-walked
and re-opened later.

Follow up fix for #5859.

PiperOrigin-RevId: 371509392

1 files changed, 14 insertions, 8 deletions

diff --git a/pkg/sentry/fsimpl/gofer/filesystem.go b/pkg/sentry/fsimpl/gofer/filesystem.go
index c1c9ec008..97ce80853 100644
--- a/pkg/sentry/fsimpl/gofer/filesystem.go
+++ b/pkg/sentry/fsimpl/gofer/filesystem.go
@@ -117,6 +117,17 @@ func appendDentry(ds *[]*dentry, d *dentry) *[]*dentry {
 	return ds
 }
 
+// Precondition: !parent.isSynthetic() && !child.isSynthetic().
+func appendNewChildDentry(ds **[]*dentry, parent *dentry, child *dentry) {
+	// The new child was added to parent and took a ref on the parent (hence
+	// parent can be removed from cache). A new child has 0 refs for now. So
+	// checkCachingLocked() should be called on both. Call it first on the parent
+	// as it may create space in the cache for child to be inserted - hence
+	// avoiding a cache eviction.
+	*ds = appendDentry(*ds, parent)
+	*ds = appendDentry(*ds, child)
+}
+
 // Preconditions: ds != nil.
 func putDentrySlice(ds *[]*dentry) {
 	// Allow dentries to be GC'd.
@@ -258,11 +269,7 @@ func (fs *filesystem) getChildLocked(ctx context.Context, parent *dentry, name s
 		return nil, err
 	}
 	parent.cacheNewChildLocked(child, name)
-	// For now, child has 0 references, so our caller should call
-	// child.checkCachingLocked(). parent gained a ref so we should also call
-	// parent.checkCachingLocked() so it can be removed from the cache if needed.
-	*ds = appendDentry(*ds, child)
-	*ds = appendDentry(*ds, parent)
+	appendNewChildDentry(ds, parent, child)
 	return child, nil
 }
 
@@ -1114,7 +1121,6 @@ func (d *dentry) createAndOpenChildLocked(ctx context.Context, rp *vfs.Resolving
 		}
 		return nil, err
 	}
-	*ds = appendDentry(*ds, child)
 	// Incorporate the fid that was opened by lcreate.
 	useRegularFileFD := child.fileType() == linux.S_IFREG && !d.fs.opts.regularFilesUseSpecialFileFD
 	if useRegularFileFD {
@@ -1138,7 +1144,7 @@ func (d *dentry) createAndOpenChildLocked(ctx context.Context, rp *vfs.Resolving
 	}
 	// Insert the dentry into the tree.
 	d.cacheNewChildLocked(child, name)
-	*ds = appendDentry(*ds, d)
+	appendNewChildDentry(ds, d, child)
 	if d.cachedMetadataAuthoritative() {
 		d.touchCMtime()
 		d.dirents = nil
@@ -1333,9 +1339,9 @@ func (fs *filesystem) RenameAt(ctx context.Context, rp *vfs.ResolvingPath, oldPa
 	// parent isn't actually changing.
 	if oldParent != newParent {
 		oldParent.decRefNoCaching()
-		ds = appendDentry(ds, oldParent)
 		newParent.IncRef()
 		ds = appendDentry(ds, newParent)
+		ds = appendDentry(ds, oldParent)
 		if renamed.isSynthetic() {
 			oldParent.syntheticChildren--
 			newParent.syntheticChildren++