path: root/init
diff options
authorRasmus Villemoes <linux@rasmusvillemoes.dk>2021-05-06 18:05:42 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2021-05-07 00:26:33 -0700
commite7cb072eb988e46295512617c39d004f9e1c26f8 (patch)
tree8a00475424eb11db78cb0d7599407f72aa107b75 /init
parenta065c0faacb1e472cd4e048986407d1b177373a2 (diff)
init/initramfs.c: do unpacking asynchronously
Patch series "background initramfs unpacking, and CONFIG_MODPROBE_PATH", v3. These two patches are independent, but better-together. The second is a rather trivial patch that simply allows the developer to change "/sbin/modprobe" to something else - e.g. the empty string, so that all request_module() during early boot return -ENOENT early, without even spawning a usermode helper, needlessly synchronizing with the initramfs unpacking. The first patch delegates decompressing the initramfs to a worker thread, allowing do_initcalls() in main.c to proceed to the device_ and late_ initcalls without waiting for that decompression (and populating of rootfs) to finish. Obviously, some of those later calls may rely on the initramfs being available, so I've added synchronization points in the firmware loader and usermodehelper paths - there might be other places that would need this, but so far no one has been able to think of any places I have missed. There's not much to win if most of the functionality needed during boot is only available as modules. But systems with a custom-made .config and initramfs can boot faster, partly due to utilizing more than one cpu earlier, partly by avoiding known-futile modprobe calls (which would still trigger synchronization with the initramfs unpacking, thus eliminating most of the first benefit). This patch (of 2): Most of the boot process doesn't actually need anything from the initramfs, until of course PID1 is to be executed. So instead of doing the decompressing and populating of the initramfs synchronously in populate_rootfs() itself, push that off to a worker thread. This is primarily motivated by an embedded ppc target, where unpacking even the rather modest sized initramfs takes 0.6 seconds, which is long enough that the external watchdog becomes unhappy that it doesn't get attention soon enough. By doing the initramfs decompression in a worker thread, we get to do the device_initcalls and hence start petting the watchdog much sooner. Normal desktops might benefit as well. On my mostly stock Ubuntu kernel, my initramfs is a 26M xz-compressed blob, decompressing to around 126M. That takes almost two seconds: [ 0.201454] Trying to unpack rootfs image as initramfs... [ 1.976633] Freeing initrd memory: 29416K Before this patch, these lines occur consecutively in dmesg. With this patch, the timestamps on these two lines is roughly the same as above, but with 172 lines inbetween - so more than one cpu has been kept busy doing work that would otherwise only happen after the populate_rootfs() finished. Should one of the initcalls done after rootfs_initcall time (i.e., device_ and late_ initcalls) need something from the initramfs (say, a kernel module or a firmware blob), it will simply wait for the initramfs unpacking to be done before proceeding, which should in theory make this completely safe. But if some driver pokes around in the filesystem directly and not via one of the official kernel interfaces (i.e. request_firmware*(), call_usermodehelper*) that theory may not hold - also, I certainly might have missed a spot when sprinkling wait_for_initramfs(). So there is an escape hatch in the form of an initramfs_async= command line parameter. Link: https://lkml.kernel.org/r/20210313212528.2956377-1-linux@rasmusvillemoes.dk Link: https://lkml.kernel.org/r/20210313212528.2956377-2-linux@rasmusvillemoes.dk Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk> Reviewed-by: Luis Chamberlain <mcgrof@kernel.org> Cc: Jessica Yu <jeyu@kernel.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Nick Desaulniers <ndesaulniers@google.com> Cc: Takashi Iwai <tiwai@suse.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'init')
2 files changed, 38 insertions, 1 deletions
diff --git a/init/initramfs.c b/init/initramfs.c
index d677e8e717f1..af27abc59643 100644
--- a/init/initramfs.c
+++ b/init/initramfs.c
@@ -1,5 +1,6 @@
// SPDX-License-Identifier: GPL-2.0
#include <linux/init.h>
+#include <linux/async.h>
#include <linux/fs.h>
#include <linux/slab.h>
#include <linux/types.h>
@@ -541,6 +542,14 @@ static int __init keepinitrd_setup(char *__unused)
__setup("keepinitrd", keepinitrd_setup);
+static bool __initdata initramfs_async = true;
+static int __init initramfs_async_setup(char *str)
+ strtobool(str, &initramfs_async);
+ return 1;
+__setup("initramfs_async=", initramfs_async_setup);
extern char __initramfs_start[];
extern unsigned long __initramfs_size;
#include <linux/initrd.h>
@@ -658,7 +667,7 @@ static void __init populate_initrd_image(char *err)
#endif /* CONFIG_BLK_DEV_RAM */
-static int __init populate_rootfs(void)
+static void __init do_populate_rootfs(void *unused, async_cookie_t cookie)
/* Load the built in initramfs */
char *err = unpack_to_rootfs(__initramfs_start, __initramfs_size);
@@ -693,6 +702,33 @@ done:
initrd_end = 0;
+static ASYNC_DOMAIN_EXCLUSIVE(initramfs_domain);
+static async_cookie_t initramfs_cookie;
+void wait_for_initramfs(void)
+ if (!initramfs_cookie) {
+ /*
+ * Something before rootfs_initcall wants to access
+ * the filesystem/initramfs. Probably a bug. Make a
+ * note, avoid deadlocking the machine, and let the
+ * caller's access fail as it used to.
+ */
+ pr_warn_once("wait_for_initramfs() called before rootfs_initcalls\n");
+ return;
+ }
+ async_synchronize_cookie_domain(initramfs_cookie + 1, &initramfs_domain);
+static int __init populate_rootfs(void)
+ initramfs_cookie = async_schedule_domain(do_populate_rootfs, NULL,
+ &initramfs_domain);
+ if (!initramfs_async)
+ wait_for_initramfs();
return 0;
diff --git a/init/main.c b/init/main.c
index dd11bfd10ead..11d34ccf5786 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1561,6 +1561,7 @@ static noinline void __init kernel_init_freeable(void)
+ wait_for_initramfs();

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