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Date:   Mon, 24 Oct 2022 10:00:20 -0400
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Subject: Re: [PATCH v2 2/2] module: Merge same-name module load requests
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To:     Petr Pavlu <petr.pavlu@suse.com>,
        Luis Chamberlain <mcgrof@kernel.org>
Cc:     pmladek@suse.com, linux-modules@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20220919123233.8538-1-petr.pavlu@suse.com>
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From:   Prarit Bhargava <prarit@redhat.com>
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On 10/24/22 08:37, Petr Pavlu wrote:
> On 10/18/22 21:53, Prarit Bhargava wrote:
>> Quoting from the original thread,
>>
>>>
>>> Motivation for this patch is to fix an issue observed on larger machines with
>>> many CPUs where it can take a significant amount of time during boot to run
>>> systemd-udev-trigger.service. An x86-64 system can have already intel_pstate
>>> active but as its CPUs can match also acpi_cpufreq and pcc_cpufreq, udev will
>>> attempt to load these modules too. The operation will eventually fail in the
>>> init function of a respective module where it gets recognized that another
>>> cpufreq driver is already loaded and -EEXIST is returned. However, one uevent
>>> is triggered for each CPU and so multiple loads of these modules will be
>>> present. The current code then processes all such loads individually and
>>> serializes them with the barrier in add_unformed_module().
>>>
>>
>> The way to solve this is not in the module loading code, but in the udev
>> code by adding a new event or in the userspace which handles the loading
>> events.
>>
>> Option 1)
>>
>> Write/modify a udev rule to to use a flock userspace file lock to
>> prevent repeated loading.  The problem with this is that it is still
>> racy and still consumes CPU time repeated load the ELF header and,
>> depending on the system (ie a large number of cpus) would still cause a
>> boot delay.  This would be better than what we have and is worth looking
>> at as a simple solution.  I'd like to see boot times with this change,
>> and I'll try to come up with a measurement on a large CPU system.
> 
> It is not immediately clear to me how this can be done as a udev rule. You
> mention that you'll try to test this on a large CPU system. Does it mean that
> you have a prototype implemented already? If yes, could you please share it?
> 

Hi Petr,

Sorry, I haven't had a chance to actually test this out but I see this 
problem with the acpi_cpufreq and other multiple-cpu drivers which load 
once per logical cpu.  I was thinking of adding a udev rule like:

ACTION!="add", GOTO="acpi_cpufreq_end"

# I may have to add CPU modaliases here to get this to work correctly
ENV{MODALIAS}=="acpi:ACPI0007:", GOTO="acpi_cpufreq_start"

GOTO="acpi_cpufreq_start"
GOTO="acpi_cpufreq_end"

LABEL="acpi_cpufreq_start"

ENV{DELAY_MODALIAS}="$env{MODALIAS}"
ENV{MODALIAS}=""
PROGRAM="/bin/sh -c flock -n /tmp/delay_acpi_cpufreq sleep 2'", 
RESULT=="", RUN{builtin}+="kmod load $env{DELAY_MODALIAS}"

LABEL="acpi_cpufreq_end"


> My reading is that one would need to update the "MODALIAS" rule in
> 80-drivers.rules [1] to do this locking. However, that just collects
> 'kmod load' (builtin) for udev to execute after all rules are processed. It
> would then be required to synchronize udev workers to prevent repeated
> loading?
> 

Yes, that would be the case.

>> Option 2)
>>
>> Create a new udev action, "add_once" to indicate to userspace that the
>> module only needs to be loaded one time, and to ignore further load
>> requests.  This is a bit tricky as both kernel space and userspace would
>> have be modified.  The udev rule would end up looking very similar to
>> what we now.
>>
>> The benefit of option 2 is that driver writers themselves can choose
>> which drivers should issue "add_once" instead of add.  Drivers that are
>> known to run on all devices at once would call "add_once" to only issue
>> a single load.
> 
> On the device event side, I more wonder if it would be possible to avoid tying
> up cpufreq and edac modules to individual CPU devices. Maybe their loading
> could be attached to some platform device, even if it means introducing an
> auxiliary device for this purpose? I need to look a bit more into this idea.

That's an interesting idea and something I had not considered.  Creating 
a virtual device and loading against that device would be much better 
(easier?) of a solution.

P.

> 
> [1] https://github.com/systemd/systemd/blob/4856f63846fc794711e1b8ec970e4c56494cd320/rules.d/80-drivers.rules
> 
> Thanks,
> Petr
>