efisubsys_init takes more than a few milliseconds

[Paul Menzel]

Dear Ard,


According to `initcall_debug`, `efisubsys_init` takes more than a few 
milliseconds to execute on a Dell XPS 13 9370 (Intel(R) Core(TM) 
i7-8550U CPU @ 1.80GHz).

> ```
> [â]
> [    0.144474] calling  efisubsys_init+0x0/0x2cf @ 1
> [    0.144474] Registered efivars operations
> [    0.173690] initcall efisubsys_init+0x0/0x2cf returned 0 after 27343 usecs
> [â]
> ```

To get a vanilla Linux kernel to boot in well under one second, itâd be 
nice if the time could be improved. Do you know, why it takes so long?

According to `bootgraph.py` from pm-graph [1][2] it takes even a little 
longer.

> efisubsys_init: start=690.841, end=720.493, length(w/o overhead)=31.250 ms, return=0

There are several dozen calls to `virt_efi_get_next_variable()` all but 
one taking around 0.335 ms. This path needs to be optimized. Is that 
possible?

To reproduce this, clone the pm-graph repository [2], use `sudo 
./bootgraph.py -f -fstat -maxdepth 10 -manual` to see what to add to 
`/boot/grub/grub.cfg`. Then reboot, and execute `sudo ./bootgraph.py -f 
-fstat -maxdepth 10`.

If your system is powerful enough, you can use a higher maximum depth. I 
didnât get around how `-cgfilter` works to get smaller HTML files.


Kind regards,

Paul


[1] https://01.org/suspendresume
[2] https://github.com/01org/pm-graph