logind: How to process enumerate buttons in parallel?

[lennart@xxxxxxxxxxxxxx (Lennart Poettering)]

On Do, 12.07.18 16:51, Paul Menzel (pmenzel+systemd-devel at molgen.mpg.de) wrote:

> Dear systemd folks,
> 
> 
> 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.

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

Lennart

-- 
Lennart Poettering, Red Hat