Messages in this thread | | | Date | Fri, 1 May 2009 13:38:13 +0100 | Subject | Re: [PATCH] driver-core: devtmpfs - driver core maintained /dev tmpfs | From | Alan Jenkins <> |
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On 5/1/09, Kay Sievers <kay.sievers@vrfy.org> wrote: > On Fri, May 1, 2009 at 12:19, Alan Jenkins > <sourcejedi.lkml@googlemail.com> wrote: >> On 4/30/09, Kay Sievers <kay.sievers@vrfy.org> 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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