Re: [PATCH v3 1/2] init/initramfs.c: do unpacking asynchronously

[Luis Chamberlain <mcgrof@kernel.org>]

On Sat, Mar 13, 2021 at 10:25:27PM +0100, Rasmus Villemoes wrote:
> 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.
> 
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>

Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>

  Luis