linux-kernel - Re: [PATCH RFC 0/3] Static calls

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Date:   Mon, 12 Nov 2018 06:02:41 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...LAB.COM>,
        Borislav Petkov <bp@...en8.de>, Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH RFC 0/3] Static calls


* Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> On Fri, Nov 09, 2018 at 08:28:11AM +0100, Ingo Molnar wrote:
> > > - I'm not sure about the objtool approach.  Objtool is (currently)
> > >   x86-64 only, which means we have to use the "unoptimized" version
> > >   everywhere else.  I may experiment with a GCC plugin instead.
> > 
> > I'd prefer the objtool approach. It's a pretty reliable first-principles 
> > approach while GCC plugin would have to be replicated for Clang and any 
> > other compilers, etc.
> 
> The benefit of a plugin is that we'd only need two of them: GCC and
> Clang.  And presumably, they'd share a lot of code.
> 
> The prospect of porting objtool to all architectures is going to be much
> more of a daunting task (though we are at least already considering it
> for some arches).

Which architectures would benefit from ORC support the most?

I really think that hard reliance on GCC plugins is foolish - but maybe 
Clang's plugin infrastructure is a guarantee that it remains a sane and 
usable interface.

> > I'd be very happy with a demonstrated paravirt optimization already - 
> > i.e. seeing the before/after effect on the vmlinux with an x86 distro 
> > config.
> > 
> > All major Linux distributions enable CONFIG_PARAVIRT=y and 
> > CONFIG_PARAVIRT_XXL=y on x86 at the moment, so optimizing it away as much 
> > as possible in the 99.999% cases where it's not used is a primary 
> > concern.
> 
> For paravirt, I was thinking of it as more of a cleanup than an
> optimization.  The paravirt patching code already replaces indirect
> branches with direct ones -- see paravirt_patch_default().
> 
> Though it *would* reduce the instruction footprint a bit, as the 7-byte
> indirect calls (later patched to 5-byte direct + 2-byte nop) would
> instead be 5-byte direct calls to begin with.

Yes.

> > All other usecases are bonus, but it would certainly be interesting to 
> > investigate the impact of using these APIs for tracing: that too is a 
> > feature enabled everywhere but utilized only by a small fraction of Linux 
> > users - so literally every single cycle or instruction saved or hot-path 
> > shortened is a major win.
> 
> With retpolines, and with tracepoints enabled, it's definitely a major
> win.  Steve measured an 8.9% general slowdown on hackbench caused by
> retpolines.

How much of that slowdown is reversed?

> But with tracepoints disabled, I believe static jumps are used, which
> already minimizes the impact on hot paths.

Yeah.

Thanks,

	Ing