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SubjectRe: clang asm-goto support (Was Re: [PATCH v2] x86/retpoline: Add clang support)

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Wed, Feb 14, 2018 at 09:52:59AM +0000, Yatsina, Marina wrote:
> > Hi Peter,
> >
> > When I started the original thread last year I was in favor of adding
> > "asm goto" and didn't understand why it wasn't done by that time. The
> > feedback I got is that this feature (optimizing tracepoints) is very
> > useful and that we do want it in llvm, but perhaps there's a cleaner
> > way of implementing than "asm goto". An alternative suggestion arose
> > as well.
>
> So it's far more than just tracepoints. We use it all over the kernel to
> do runtime branch patching.
>
> One example is avoiding the scheduler preemption callbacks if we know
> there are no users. This shaves a few % off a context switch
> micro-bench.
>
> But it is really _all_ over the place.

To quantify it: I just performed a test build of a Linux distro kernel config
(Fedora x86-64), and counted the number of callsites that use 'asm goto'
functionality with the v4.15 kernel (including drivers).

The results:

Linux distro | !CONFIG_TRACING
-----------------------------------------------------------------------------
total # of functions : 191,567 | 184,443
total # of instructions : 14,251,355 | 13,526,112
-----------------------------------------------------------------------------
total # of spin_lock*() calls : 25,246 | 25,177
total # of mutex_lock*() calls : 13,062 | 12,861
total # of kmalloc*() calls : 5,148 | 5,118
-----------------------------------------------------------------------------
total # of 'asm goto' usage sites : 34,851 | 31,059
total # of 'asm goto' using functions : 18,209 | 16,089
-----------------------------------------------------------------------------
percent of kernel functions using 'asm goto' : 9.5% | 8.7%
-----------------------------------------------------------------------------


( Note: I added the 'tracing disabled' column only to demonstrate how widely
'asm goto' is used beyond tracing - but since *all* major Linux
distributions have tracing enabled it's the first column that matters in
practice. )

So 'asm goto' is a major, major compiler feature the Linux kernel relies on:
in fact in the Linux kernel there's more 'asm goto' usage sites than there are
spin-lock critical sections!

Thanks,

Ingo

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