linux-kernel - Re: [PATCH v2 1/2] init/initramfs.c: allow asynchronous unpacking

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Date:   Tue, 9 Mar 2021 23:51:57 +0100
From:   Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Luis Chamberlain <mcgrof@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jessica Yu <jeyu@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Jonathan Corbet <corbet@....net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2 1/2] init/initramfs.c: allow asynchronous unpacking

On 09/03/2021 23.16, Linus Torvalds wrote:
> On Tue, Mar 9, 2021 at 1:17 PM Rasmus Villemoes
> <linux@...musvillemoes.dk> wrote:
>>
>> So add an initramfs_async= kernel parameter, allowing the main init
>> process to proceed to handling device_initcall()s without waiting for
>> populate_rootfs() to finish.
> 
> Oh, and a completely unrelated second comment about this: some of the
> initramfs population code seems to be actively written to be slow.
> 
> For example, I'm not sure why that write_buffer() function uses an
> array of indirect function pointer actions. Even ignoring the whole
> "speculation protections make that really slow" issue that came later,
> it seems to always have been actively (a) slower and (b) more complex.
> 
[...]
> Is that likely to be a big part of the costs? No. I assume it's the
> decompression and the actual VFS operations. 

Yes, I have been doing some simple measurements, simply by decompressing
the blob in userspace and comparing to the time to that used by
populate_rootfs(). For both the 6M lz4-compressed blob on my ppc target
and the 26M xz-compressed blob on my laptop, the result is that the
decompression itself accounts for the vast majority of the time - and
for ppc in particular, I don't think there's any spectre slowdown.

So I haven't dared looking into changing the unpack implementation since
it doesn't seem it could buy that much.

Rasmus