Re: [PATCH] driver-core: devtmpfs - driver core maintained /dev tmpfs

[Alan Jenkins]

On 5/1/09, Kay Sievers <kay.sievers@xxxxxxxx> wrote:
> On Fri, May 1, 2009 at 12:19, Alan Jenkins
> <sourcejedi.lkml@xxxxxxxxxxxxxx> wrote:
>> On 4/30/09, Kay Sievers <kay.sievers@xxxxxxxx> wrote:
>
>>> With the kernel populated /dev, existing initramfs or kernel-mount
>>> bootup logic can be optimized to be more efficient, and not to require a
>>> full coldplug run, which is currently needed to bootstrap the inital
>>> /dev directory content, before continuing bringing up the rest of
>>> the system. There will be no missed events to replay, because /dev is
>>> available before the first kernel device is registered with the core.
>>> A coldplug run can take, depending on the speed of the system and the
>>> amount of devices which need to be handled, from one to several seconds.
>>
>> Aren't you overeaching in your claims here?  I'm sure you can't avoid
>> at least one coldplug run on a contemporary general purpose system,
>
> You still need coldplug, sure. But distros have 2 full coldplugs
> today, one in initramfs, one in the rootfs.
>
>> because you lose so much of the functionality provided by udev.
>
> No, udev will do the same thing as it does today, it will not be
> different, it's just that the colplug can run in parallel with other
> stuff, and the initramfs coldplug can be much simplified, or even
> avoided.

Ok, I can see there's no problem there, given that coldplug still
happens eventually.

>> I'm
>> sure of that, but it would be nice if you could address it in the
>> changelog.  And modern initramfs' require udev RUN rules to read UUIDs
>> and set up LVM.
>
> A simple uuid/label lookup will not need a full coldplug run in
> initramfs. You just need to start udev, and wait for the links to show
> up.Possibly running triggering block events, but not the hundreds of
> other devices.

Assuming you have all the necessary modules loaded!  I guess that's
easy enough though

find /sys -name modalias -exec cat \{\} \; | xargs -n 1 modprobe -Qa

bonus: increase "-n" for reduced modprobe fork overhead.

>> I'm loving this for embedded, init=/bin/sh, and rescue floppies :-).
>> But I can't understand how you plan to use this as an optimisation.
>>
>> And - I'm sure you must have considered this in a moment of madness -
>> do you know why we couldn't just start _udev_ "before the first kernel
>> device is registered with the core"?
>
> I think it's pretty obvious, that this is the fastest you can get for
> /dev. And it's not about the time when udev is started, its about
> doing the stuff when the devices are created, and not to need to redo
> it at bootup, or do it even twice.

Ok, yes.  I can see that cutting initramfs coldplug down to size is a win.

I thought it was a useful comparison.  Start udev early enough, and
you could avoid re-doing absolutely anything.  Thinking about, the
reasons it doesn't work are

a) running before /dev/null and /dev/console requires hacks
b) it requires an initramfs
c) it pulls everything that hooks into or otherwise affects udev into
the initramfs; that's much more than we have at present, and a bigger
initramfs' can only make bootup _slower_.

Regards
Alan
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