linux-kernel - Re: [PATCH 00/12] x86: Rewrite 64-bit syscall code

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Date:	Mon, 7 Dec 2015 21:42:27 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Brian Gerst <brgerst@...il.com>,
	Borislav Petkov <bp@...en8.de>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 00/12] x86: Rewrite 64-bit syscall code

On Mon, Dec 7, 2015 at 8:42 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Andy Lutomirski <luto@...capital.net> wrote:
>
>> On Mon, Dec 7, 2015 at 1:51 PM, Andy Lutomirski <luto@...nel.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?  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?

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.)

--Andy
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