smp: respect cgroup CPU limits for appliance SMP

mlutils/unix_utils.ml

@@ -89,3 +89,28 @@ module Sysconf = struct
   external nr_processors_online : unit -> int =
     "guestfs_int_mllib_sysconf_nr_processors_online" [@@noalloc]
 end
+
+module Cgroup = struct
+  let v2_cpus () =
+    let file = "/sys/fs/cgroup/cpu.max" in
+    if Sys.file_exists file then
+      try
+        let line = read_first_line_from_file file in
+        if String.starts_with ~prefix:"max" line then
+          None
+        else
+          let quota, period =
+            Scanf.sscanf line "%Ld %Ld" (fun q p -> (q, p)) in
+          if period > 0L then
+            Some (max 1 (Int64.to_int (Int64.div quota period)))
+          else None
+      with
+      | Scanf.Scan_failure _ | Failure _ | End_of_file -> None
+    else
+      None
+
+  let nr_cpus_available () =
+    match v2_cpus () with
+    | Some cpus -> cpus
+    | None -> Sysconf.nr_processors_online ()
+end

mlutils/unix_utils.mli

@@ -130,3 +130,21 @@ module Sysconf : sig
       Note this never fails.  In case we cannot get the number of
       cores it returns 1. *)
 end
+
+module Cgroup : sig
+  (** Functions to read CPU limits from cgroup filesystems. *)
+
+  val v2_cpus : unit -> int option
+  (** [v2_cpus ()] reads the cgroup v2 CPU quota from
+      [/sys/fs/cgroup/cpu.max] and returns the number of CPUs
+      allocated (quota / period), or [None] if the file does not
+      exist, the quota is "max" (unlimited), or the file cannot
+      be parsed.  The kernel stores quota and period as [long]
+      values in microseconds, so we parse them as [Int64]. *)
+
+  val nr_cpus_available : unit -> int
+  (** [nr_cpus_available ()] returns the number of CPUs available,
+      taking into account cgroup v2 CPU limits.
+      Falls back to {!Sysconf.nr_processors_online} if no cgroup
+      limits are set. *)
+end