logind: How to process enumerate buttons in parallel?

[pmenzel+systemd-devel@xxxxxxxxxxxxx (Paul Menzel)]

Dear Lennart,


Thank you for your response.


On 07/13/18 17:12, Lennart Poettering wrote:
> On Do, 12.07.18 16:51, Paul Menzel wrote:

>> Trying to decrease the start-up time, I noticed that `systemd-logind`
>> probes devices in serial(?) (instead of parallel).
>>
>> From `manager_enumerate_buttons(Manager *m)` in `src/login/logind.c`:
>>
>> ```
>>         udev_list_entry_foreach(item, first) {
>>                 _cleanup_(udev_device_unrefp) struct udev_device *d = NULL;
>>                 int k;
>>
>>                 d = udev_device_new_from_syspath(m->udev, udev_list_entry_get_name(item));
>>                 if (!d)
>>                         return -ENOMEM;
>>
>>                 k = manager_process_button_device(m, d);
>>                 if (k < 0)
>>                         r = k;
>>         }
>> ```
>>
>> The macro is defined as below:
>>
>> ```
>> /**
>>  * udev_list_entry_foreach:
>>  * @list_entry: entry to store the current position
>>  * @first_entry: first entry to start with
>>  *
>>  * Helper to iterate over all entries of a list.
>>  */
>> #define udev_list_entry_foreach(list_entry, first_entry) \
>>         for (list_entry = first_entry; \
>>              list_entry != NULL; \
>>              list_entry = udev_list_entry_get_next(list_entry))
>> ```
>>
>> Is there a way to do that in parallel?
> 
> systemd components are generally single threaded. Probing input
> devices from systemd is unlikely to be a performance problem. I mean,
> we don't process 10K devices here, but usually not more than 25 or
> so... Also the code isn't really CPU or IO bound here, it just does a
> few open() and ioctl() calls mostly.

By the way, below are the timings from the journal.

```
$ journalctl -o short-precise -u systemd-logind.service -b
-- Logs begin at Sat 2017-12-16 23:28:06 CET, end at Mon 2018-07-16 08:23:11 CEST. --
Jul 16 08:22:54.006190 kodi systemd[1]: Starting Login Service...
Jul 16 08:22:54.103445 kodi systemd-logind[169]: New seat seat0.
Jul 16 08:22:54.163713 kodi systemd[1]: Started Login Service.
Jul 16 08:22:54.312955 kodi systemd-logind[169]: Watching system buttons on /dev/input/event1 (Power Button)
Jul 16 08:22:54.514965 kodi systemd-logind[169]: Watching system buttons on /dev/input/event0 (Power Button)
Jul 16 08:22:54.781082 kodi systemd-logind[169]: Watching system buttons on /dev/input/event2 (AT Translated Set 2 keyboard)
```

> Before spending time on optimizing anything like this: did you did
> profiling on this, that shows clearly that this is performance
> relevant at all?

Do you have suggestions how to do that with systemd-logind?


Kind regards,

Paul

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