linux-kernel - Re: [PATCH 08/17] blkcg: shoot down blkio

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Message-ID: <20120123204352.GB21822@tango.0pointer.de>
Date:	Mon, 23 Jan 2012 21:43:52 +0100
From:	Lennart Poettering <mzxreary@...inter.de>
To:	Tejun Heo <tj@...nel.org>
Cc:	Vivek Goyal <vgoyal@...hat.com>, axboe@...nel.dk,
	ctalbott@...gle.com, rni@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/17] blkcg: shoot down blkio_groups on elevator switch

On Mon, 23.01.12 10:43, Tejun Heo (tj@...nel.org) wrote:

> 
> On Mon, Jan 23, 2012 at 01:27:45PM -0500, Vivek Goyal wrote:
> > > Why can't systemd order elevator switch before other actions?
> > 
> > Because systemd does not know. For systemd it is just launching services
> > and what services are doing is not known to systemd.
> > 
> > I think systemd does have some facilities so that services can express
> > dependency on other services and dependent service blocks on completion
> > of service it is depenent on. So may be in this case any service dealing
> > with cgroups shall have to be dependent on this service which tunes
> > the system and changes elevator.
> 
> I'm sure systemd has enough facility for expressing this dependency.
> Where this configuration belongs to is a different question tho.  I
> don't know how the tuned thing works but configurations like this are
> bound to devices and should be part of device discovery / hotplug
> sequence.  IOW, it should be something which ultimately runs off udev
> events as part of device found event.

I didn't really follow the whole discussion here, but if this is about
adjusting parameters of a block device as the block device shows up this
must necessarily happen in an udev rule (and not in a daemon watching
block devices using libudev), since you most likely need the
synchronicity: i.e. you want to avoid that normal (libudev-using)
userspace code ever sees the device before those params are written to
the device. A daemon asynchronously watching the devices with libudev
for adjusting such tunables is necessarily racy.

Lennart

-- 
Lennart Poettering - Red Hat, Inc.
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