linux-kernel - Re: efisubsys_init takes more than a few milliseconds

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Date:   Thu, 23 Aug 2018 16:50:45 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Paul Menzel <pmenzel+linux-efi@...gen.mpg.de>
Cc:     linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: efisubsys_init takes more than a few milliseconds

On 22 August 2018 at 15:18, Paul Menzel <pmenzel+linux-efi@...gen.mpg.de> wrote:
> Dear Ard,
>
>
> On 03/25/18 17:21, Paul Menzel wrote:
>
>> On 03/25/2018 09:41 AM, Paul Menzel wrote:
>>
>>> On 03/24/2018 11:35 PM, Ard Biesheuvel wrote:
>>>
>>>> On 24 March 2018 at 22:10, Paul Menzel wrote:
>>
>>>>> 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).
>>>>>
>>>>>> ```
>>>>>> […]
>>>>>> [ 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
>>>>>> […]
>>>>>> ```
>>>>>
>>>>>
>>>>> To get a vanilla Linux kernel to boot in well under one second, it’d 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?
>>>>
>>>> That depends. These are firmware calls, so to make these calls faster,
>>>> you need to modify the firmware, not the kernel.
>>>
>>> Yeah, unfortunately, no free firmware runs on this laptop, and Dell doesn’t respond to these kind of reports, as they think, it’s not important.
>>>
>>>> We may be able to make more intrusive changes to get rid of this
>>>> delay, e.g., spin up a special kernel thread, but I'd have to check in
>>>> more detail.
>>>
>>> That’d be great.
>
> On the MSI B350M MORTAR (MS-7A37) it takes even longer, almost 50 ms.
>
> ```
> [    0.204009] calling  efisubsys_init+0x0/0x30a @ 1
> [    0.204009] Registered efivars operations
> [    0.252262] initcall efisubsys_init+0x0/0x30a returned 0 after 46875 usecs
> ```
>
> If a solution could be found to move this out of the hotpath, that’d be great.
> But I am not sure, if threads can be used that early.
>

Could you narrow this down? Which call in efisubsys_init() is taking
up all this time?