linux-kernel - Re: udev: New default rule for autoloading kernel modules matching CPU modalias

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Date:	Sat, 20 Jul 2013 14:34:58 +0200
From:	Kay Sievers <kay@...y.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	systemd-devel@...ts.freedesktop.org
Subject: Re: udev: New default rule for autoloading kernel modules matching
 CPU modalias

On Sat, Jul 20, 2013 at 1:47 PM, Kay Sievers <kay@...y.org> wrote:
> On Sat, Jul 20, 2013 at 12:56 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
>
>> After a recent change present in 3.11-rc1 there is a driver, called processor,
>> that can be bound to the CPU devices whose sysfs directories are located under
>> /sys/devices/system/cpu/.  A side effect of this is that, after the driver has
>> been bound to those devices, the kernel adds DRIVER=processor to ENV for CPU
>> uevents and they don't match the default rule for autoloading modules matching
>> MODALIAS:
>>
>> DRIVER!="?*", ENV{MODALIAS}=="?*", IMPORT{builtin}="kmod load $env{MODALIAS}"
>>
>> any more.  However, there are some modules whose module aliases match specific
>> CPU features through the modalias string and those modules should be loaded
>> automatically if a compatible CPU is present.  Yet, with the processor driver
>> bound to the CPU devices the above rule is not sufficient for that, so we need
>> a new default udev rule allowing those modules to be autoloaded even if the
>> CPU devices have drivers.
>>
>> On my test systems I added the following rule for that:
>>
>> ACTION="add", SUBSYSTEM=="cpu", ENV{MODALIAS}=="?*", IMPORT{builtin}="kmod load $env{MODALIAS}"
>>
>> in a separate file, but I'm not a udev expert, so I guess it may be done in a
>> better way.
>>
>> Can you please consider adding such a rule to the default set of udev rules?
>
> The DRIVER!="?*" is an optimization which made a huge difference at
> the time we called out to /sbin/modprobe from udev rules and all the
> devices which already had a driver would not cause needless execution
> of the rather expensive modprobe binary.
>
> This obviously can't do the right thing for the completely generic,
> not bound to a driver-state, CPU modaliases when they have a driver
> now. :)
>
> These days we load the entire kmod modalias database into the udev
> process, and lookup what we need to load and load the module from
> within the udev worker process. It could be, that the optimization is
> not measurable on today's systems, if that's the case I'll remove it,
> which would be the simplest and most future proof option. Otherwise
> I'll add the CPU specific rule.
>
> I'll find that out and let you know.

I cannot see any measurable difference here that justifies that
optimization, so I removed it:
  http://cgit.freedesktop.org/systemd/systemd/commit/?id=bf7f800f2b3e93ccd1229d4717166f3a4d3af72f

Thanks,
Kay
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