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

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Date:   Sat, 13 Mar 2021 14:13:08 +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 11/03/2021 19.02, Linus Torvalds wrote:
> On Wed, Mar 10, 2021 at 5:45 PM Rasmus Villemoes
> <linux@...musvillemoes.dk> wrote:
>>
>> Hm, gcc does elide the test of the return value, but jumps back to a
>> place where it always loads state from its memory location and does the
>> whole switch(). To get it to jump directly to the code implementing the
>> various do_* helpers it seems one needs to avoid that global variable
>> and instead return the next state explicitly. The below boots, but I
>> still can't see any measurable improvement on ppc.
> 
> Ok. That's definitely the right way to do efficient statemachines that
> the compiler can actually generate ok code for, but if you can't
> measure the difference I guess it isn't even worth doing.

Just for good measure, I now got around to test on x86 as well, where I
thought the speculation stuff might make a difference. However, the
indirect calls through the actions[] array don't actually hurt due to
__noinitretpoline, and even removing that from the __init definition, I
only see about 1.5% difference with that state machine patch applied.

So it doesn't seem worth pursuing. I'll send v3 of the async patches
shortly.

Rasmus