ueventd: parallelize uevent handling

fork() subprocesses to handle uevents in parallel.

This reduces coldboot time on bullhead from ~446ms to ~230ms.
This reduces coldboot time on sailfish from ~690ms to ~360ms.
This reduces coldboot time on ryu from ~187ms to ~122ms.

Bug: 33785894

Test: boot bullhead x40, observe no major differences in /dev and /sys
Test: boot sailfish x40, observe no major differences in /dev and /sys
Test: boot ryu x40, observe no major differences in /dev and /sys
Test: boottime tests on bullhead and sailfish
Test: init unit tests

Change-Id: Ie2f63e000b8af78d187477d31fe109f20304d749

1 files changed, 22 insertions, 6 deletions

diff --git a/init/firmware_handler.cpp b/init/firmware_handler.cpp
index 1471aeb47..844c6051f 100644
--- a/init/firmware_handler.cpp
+++ b/init/firmware_handler.cpp
@@ -18,6 +18,7 @@
 
 #include <fcntl.h>
 #include <sys/sendfile.h>
+#include <sys/wait.h>
 #include <unistd.h>
 
 #include <string>
@@ -103,14 +104,29 @@ void HandleFirmwareEvent(const Uevent& uevent) {
     if (uevent.subsystem != "firmware" || uevent.action != "add") return;
 
     // Loading the firmware in a child means we can do that in parallel...
-    // (We ignore SIGCHLD rather than wait for our children.)
+    // We double fork instead of waiting for these processes.
     pid_t pid = fork();
+    if (pid == -1) {
+        PLOG(ERROR) << "could not fork to process firmware event for " << uevent.firmware;
+        return;
+    }
+
     if (pid == 0) {
-        Timer t;
-        ProcessFirmwareEvent(uevent);
-        LOG(INFO) << "loading " << uevent.path << " took " << t;
+        pid = fork();
+        if (pid == -1) {
+            PLOG(ERROR) << "could not fork a sceond time to process firmware event for "
+                        << uevent.firmware;
+            _exit(EXIT_FAILURE);
+        }
+        if (pid == 0) {
+            Timer t;
+            ProcessFirmwareEvent(uevent);
+            LOG(INFO) << "loading " << uevent.path << " took " << t;
+            _exit(EXIT_SUCCESS);
+        }
+
         _exit(EXIT_SUCCESS);
-    } else if (pid == -1) {
-        PLOG(ERROR) << "could not fork to process firmware event for " << uevent.firmware;
     }
+
+    waitpid(pid, nullptr, 0);
 }