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SubjectRe: [PATCH 00/12] x86: Rewrite 64-bit syscall code

* Andy Lutomirski <luto@amacapital.net> wrote:

> On Mon, Dec 7, 2015 at 8:42 PM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Andy Lutomirski <luto@amacapital.net> wrote:
> >
> >> On Mon, Dec 7, 2015 at 1:51 PM, Andy Lutomirski <luto@kernel.org> wrote:
> >>
> >> > This is kind of like the 32-bit and compat code, except that I preserved the
> >> > fast path this time. I was unable to measure any significant performance
> >> > change on my laptop in the fast path.
> >> >
> >> > What do you all think?
> >>
> >> For completeness, if I zap the fast path entirely (see attached), I lose 20
> >> cycles (148 cycles vs 128 cycles) on Skylake. Switching between movq and pushq
> >> for stack setup makes no difference whatsoever, interestingly. I haven't tried
> >> to figure out exactly where those 20 cycles go.
> >
> > So I asked for this before, and I'll do so again: could you please stick the cycle
> > granular system call performance test into a 'perf bench' variant so that:
> >
> > 1) More people can run it all on various pieces of hardware and help out quantify
> > the patches.
> >
> > 2) We can keep an eye on not regressing base system call performance in the
> > future, with a good in-tree testcase.
> >
>
> Is it okay if it's not particularly shiny or modular? [...]

Absolutely!

> [...] The tool I'm using is here:
>
> https://git.kernel.org/cgit/linux/kernel/git/luto/misc-tests.git/tree/tight_loop/perf_self_monitor.c
>
> and I can certainly stick it into 'perf bench' pretty easily. Can I
> leave making it into a proper library to some future contributor?

Sure - 'perf bench' tests aren't librarized generally - the goal is to make it
easy to create a new measurement.

> It's actually decently fancy. It allocates a perf self-monitoring
> instance that counts cycles, and then it takes a bunch of samples and
> discards any that flagged a context switch. It does some very
> rudimentary statistics on the rest. It's utterly devoid of a fancy
> UI, though.
>
> It works very well on native, and it works better than I had expected
> under KVM. (KVM traps RDPMC because neither Intel nor AMD has seen
> fit to provide any sensible way to virtualize RDPMC without exiting.)

Sounds fantastic to me!

Thanks,

Ingo


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