Reduce the cost of sysinfo(2).

- sysinfo(2) does not actually require a fine-grained breakdown of memory
  usage. Accordingly, instead of calling pgalloc.MemoryFile.UpdateUsage() to
  update the sentry's fine-grained memory accounting snapshot, just use
  pgalloc.MemoryFile.TotalUsage() (which is a single fstat(), and therefore far
  cheaper).

- Use the number of threads in the root PID namespace (i.e. globally) rather
  than in the task's PID namespace for consistency with Linux (which just reads
  global variable nr_threads), and add a new method to kernel.PIDNamespace to
  allow this to be read directly from an underlying map rather than requiring
  the allocation and population of an intermediate slice.

PiperOrigin-RevId: 336353100

1 files changed, 7 insertions, 0 deletions

diff --git a/pkg/sentry/kernel/threads.go b/pkg/sentry/kernel/threads.go
index 5ae5906e8..fdadb52c0 100644
--- a/pkg/sentry/kernel/threads.go
+++ b/pkg/sentry/kernel/threads.go
@@ -265,6 +265,13 @@ func (ns *PIDNamespace) Tasks() []*Task {
 	return tasks
 }
 
+// NumTasks returns the number of tasks in ns.
+func (ns *PIDNamespace) NumTasks() int {
+	ns.owner.mu.RLock()
+	defer ns.owner.mu.RUnlock()
+	return len(ns.tids)
+}
+
 // ThreadGroups returns a snapshot of the thread groups in ns.
 func (ns *PIDNamespace) ThreadGroups() []*ThreadGroup {
 	return ns.ThreadGroupsAppend(nil)