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

[Ingo Molnar]

* Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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