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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH 2/2] module: add support to avoid duplicates early on load
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On 02.06.23 18:06, Linus Torvalds wrote:
> On Fri, Jun 2, 2023 at 11:20 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> What concerns me a bit, is that on the patched kernel we seem to hit more cases where
>> boot takes much longer (in both kernel configs).
> 
> So it potentially serializes the loads to the same file more, but in
> the process uses much less memory (since the ones waiting will not
> have done any of the "load file contents and uncompress them"). So
> it's a bit of a trade-off.

I have the feeling that -- on this system -- it's some inaccurate 
accounting of firmware+loader times to the kernel startup time. Combined 
with some other noise. Especially the firmware loading time seems to be 
fairly randomized.

I guess what we care about regarding module loading is the 
initrd+userspace loading times, and they are fairly stable. But we 
mostly care about udev.

So let's look only at "systemd-udev" services:

1) !debug

a) master

5.672s systemd-udev-settle.service
  505ms systemd-udev-trigger.service
  272ms systemd-udevd.service
5.418s systemd-udev-settle.service
  487ms systemd-udev-trigger.service
  258ms systemd-udevd.service
5.707s systemd-udev-settle.service
  527ms systemd-udev-trigger.service
  273ms systemd-udevd.service
6.250s systemd-udev-settle.service
  455ms systemd-udev-trigger.service
  283ms systemd-udevd.service


b) patched

4.652s systemd-udev-settle.service
  461ms systemd-udev-trigger.service
  302ms systemd-udevd.service
4.652s systemd-udev-settle.service
  461ms systemd-udev-trigger.service
  302ms systemd-udevd.service
4.634s systemd-udev-settle.service
  444ms systemd-udev-trigger.service
  296ms systemd-udevd.service
4.745s systemd-udev-settle.service
  444ms systemd-udev-trigger.service
  273ms systemd-udevd.service


2) debug

a) master

32.806s systemd-udev-settle.service
  9.584s systemd-udev-trigger.service
   471ms systemd-udevd.service
29.901s systemd-udev-settle.service
  8.914s systemd-udev-trigger.service
   400ms systemd-udevd.service
28.640s systemd-udev-settle.service
  9.260s systemd-udev-trigger.service
   477ms systemd-udevd.service
29.498s systemd-udev-settle.service
  9.073s systemd-udev-trigger.service
   444ms systemd-udevd.service


b) patched

28.765s systemd-udev-settle.service
  8.898s systemd-udev-trigger.service
   400ms systemd-udevd.service
28.292s systemd-udev-settle.service
  8.903s systemd-udev-trigger.service
   401ms systemd-udevd.service
34.588s systemd-udev-settle.service
  8.959s systemd-udev-trigger.service
   455ms systemd-udevd.service
28.641s systemd-udev-settle.service
  8.953s systemd-udev-trigger.service
   389ms systemd-udevd.service


So except some noise, in the general case the patched version seems to 
be faster just looking at systemd-udev.

> 
> We could complicate things a bit, and let other callers return -EEXIST
> a bit earlier, but I'm not convinced it really matters.

Looking at the numbers, agreed.

> 
> Honestly, taking too long because user space does something stupid and
> wrong is not a kernel bug. Not booting because we use too much memory
> - that's problematic. But booting slowly because udev does several
> thousand unnecessary module loads is entirely on udev.

Yes.


I'll do some more experiments, but from what I can tell

Tested-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb