linux-kernel - Re: [PATCH] proc: faster /proc/*/status

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Date:	Sun, 07 Aug 2016 10:39:25 -0700
From:	Joe Perches <joe@...ches.com>
To:	Andi Kleen <andi@...stfloor.org>,
	Alexey Dobriyan <adobriyan@...il.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	viro@...iv.linux.org.uk
Subject: Re: [PATCH] proc: faster /proc/*/status

On Sun, 2016-08-07 at 09:59 -0700, Andi Kleen wrote:
> > And then what? Parsing format string is still be there.
> Perhaps could have a fast path for simple cases.
> > 
> > This is first line of profile of the first function (format_decode)
> > 
> >        │     static noinline_for_stack
> >        │     int format_decode(const char *fmt, struct printf_spec *spec)
> >        │     {
> >  10.38 │       push   %rbp			<===
> >   1.07 │       mov    %rsp,%rbp
> >   1.09 │       push   %r12
> >   4.51 │       mov    %rsi,%r12
> >   1.40 │       push   %rbx
> >   1.86 │       mov    %rdi,%rbx
> >        │       sub    $0x8,%rsp
> > 
> > It is so bloated that gcc needs to be asked to not screw up with stack
> > size.
> What happens when you drop all the noinlines for this? I assume
> this would alread make it faster. And now that we have bigger
> stacks we can likely tolerate it.

%pV recurses through these code paths.
I believe the maximum current recursion depth is 3.