Get CPU stats from the cgroup

src/libstore/build-result.hh

@@ -5,7 +5,7 @@
 
 #include <string>
 #include <chrono>
-
+#include <optional>
 
 namespace nix {
 
@@ -78,6 +78,9 @@ struct BuildResult
        was repeated). */
     time_t startTime = 0, stopTime = 0;
 
+    /* User and system CPU time the build took. */
+    std::optional<std::chrono::microseconds> cpuUser, cpuSystem;
+
     bool success()
     {
         return status == Built || status == Substituted || status == AlreadyValid || status == ResolvesToAlreadyValid;

src/libstore/build/derivation-goal.cc

@@ -869,6 +869,14 @@ void DerivationGoal::buildDone()
 
     cleanupPostChildKill();
 
+    if (buildResult.cpuUser && buildResult.cpuSystem) {
+        debug("builder for '%s' terminated with status %d, user CPU %.3fs, system CPU %.3fs",
+            worker.store.printStorePath(drvPath),
+            status,
+            ((double) buildResult.cpuUser->count()) / 1000000,
+            ((double) buildResult.cpuSystem->count()) / 1000000);
+    }
+
     bool diskFull = false;
 
     try {

src/libstore/build/local-derivation-goal.cc

@@ -137,7 +137,7 @@ void LocalDerivationGoal::killChild()
            also send a conventional kill to the child. */
         ::kill(-pid, SIGKILL); /* ignore the result */
 
-        killSandbox();
+        killSandbox(true);
 
         pid.wait();
     }
@@ -146,10 +146,14 @@ void LocalDerivationGoal::killChild()
 }
 
 
-void LocalDerivationGoal::killSandbox()
+void LocalDerivationGoal::killSandbox(bool getStats)
 {
     if (cgroup) {
-        destroyCgroup(*cgroup);
+        auto stats = destroyCgroup(*cgroup);
+        if (getStats) {
+            buildResult.cpuUser = stats.cpuUser;
+            buildResult.cpuSystem = stats.cpuSystem;
+        }
     }
 
     else if (buildUser) {
@@ -270,7 +274,7 @@ void LocalDerivationGoal::cleanupPostChildKill()
        malicious user from leaving behind a process that keeps files
        open and modifies them after they have been chown'ed to
        root. */
-    killSandbox();
+    killSandbox(true);
 
     /* Terminate the recursive Nix daemon. */
     stopDaemon();
@@ -410,7 +414,7 @@ void LocalDerivationGoal::startBuilder()
     /* Make sure that no other processes are executing under the
        sandbox uids. This must be done before any chownToBuilder()
        calls. */
-    killSandbox();
+    killSandbox(false);
 
     /* Right platform? */
     if (!parsedDrv->canBuildLocally(worker.store))

src/libstore/build/local-derivation-goal.hh

@@ -202,7 +202,7 @@ struct LocalDerivationGoal : public DerivationGoal
 
     /* Kill any processes running under the build user UID or in the
        cgroup of the build. */
-    void killSandbox();
+    void killSandbox(bool getStats);
 
     /* Create alternative path calculated from but distinct from the
        input, so we can avoid overwriting outputs (or other store paths)

src/libstore/cgroup.cc

@@ -31,13 +31,16 @@ std::map<std::string, std::string> getCgroups(const Path & cgroupFile)
     return cgroups;
 }
 
-void destroyCgroup(const Path & cgroup)
+static CgroupStats destroyCgroup(const Path & cgroup, bool returnStats)
 {
-    if (!pathExists(cgroup)) return;
+    if (!pathExists(cgroup)) return {};
+
+    if (!pathExists(cgroup + "/cgroup.procs"))
+        throw Error("'%s' is not a cgroup", cgroup);
 
     for (auto & entry : readDirectory(cgroup)) {
         if (entry.type != DT_DIR) continue;
-        destroyCgroup(cgroup + "/" + entry.name);
+        destroyCgroup(cgroup + "/" + entry.name, false);
     }
 
     int round = 1;
@@ -79,8 +82,38 @@ void destroyCgroup(const Path & cgroup)
         round++;
     }
 
+    CgroupStats stats;
+
+    if (returnStats) {
+        auto cpustatPath = cgroup + "/cpu.stat";
+
+        if (pathExists(cpustatPath)) {
+            for (auto & line : tokenizeString<std::vector<std::string>>(readFile(cpustatPath), "\n")) {
+                std::string_view userPrefix = "user_usec ";
+                if (hasPrefix(line, userPrefix)) {
+                    auto n = string2Int<uint64_t>(line.substr(userPrefix.size()));
+                    if (n) stats.cpuUser = std::chrono::microseconds(*n);
+                }
+
+                std::string_view systemPrefix = "system_usec ";
+                if (hasPrefix(line, systemPrefix)) {
+                    auto n = string2Int<uint64_t>(line.substr(systemPrefix.size()));
+                    if (n) stats.cpuSystem = std::chrono::microseconds(*n);
+                }
+            }
+        }
+
+    }
+
     if (rmdir(cgroup.c_str()) == -1)
         throw SysError("deleting cgroup '%s'", cgroup);
+
+    return stats;
+}
+
+CgroupStats destroyCgroup(const Path & cgroup)
+{
+    return destroyCgroup(cgroup, true);
 }
 
 }

src/libstore/cgroup.hh

@@ -2,13 +2,25 @@
 
 #if __linux__
 
+#include <chrono>
+#include <optional>
+
 #include "types.hh"
 
 namespace nix {
 
 std::map<std::string, std::string> getCgroups(const Path & cgroupFile);
 
-void destroyCgroup(const Path & cgroup);
+struct CgroupStats
+{
+    std::optional<std::chrono::microseconds> cpuUser, cpuSystem;
+};
+
+/* Destroy the cgroup denoted by 'path'. The postcondition is that
+   'path' does not exist, and thus any processes in the cgroup have
+   been killed. Also return statistics from the cgroup just before
+   destruction. */
+CgroupStats destroyCgroup(const Path & cgroup);
 
 }