Re: [PATCH v2 3/3] x86/mm: If INVPCID is available, use it to flush global mappings

[Andy Lutomirski <luto@amacapital.net>]

On Fri, Jan 29, 2016 at 6:26 AM, Borislav Petkov <bp@alien8.de> wrote:
> On Mon, Jan 25, 2016 at 10:37:44AM -0800, Andy Lutomirski wrote:
>> On my Skylake laptop, INVPCID function 2 (flush absolutely
>> everything) takes about 376ns, whereas saving flags, twiddling
>> CR4.PGE to flush global mappings, and restoring flags takes about
>> 539ns.
>
> FWIW, I ran your microbenchmark on the IVB laptop I have here 3 times
> and some of the numbers from each run are pretty unstable. Not that it
> means a whole lot - the thing doesn't have INVPCID support.
>
> I'm just questioning the microbenchmark and whether we should be rather
> doing those measurements with a real benchmark, whatever that means. My
> limited experience says that measuring TLB performance is hard.
>
>  ./context_switch_latency 0 thread same
>  use_xstate = 0
>  Using threads
> 1: 100000 iters at 2676.2 ns/switch
> 2: 100000 iters at 2700.2 ns/switch
> 3: 100000 iters at 2656.1 ns/switch
>
>  ./context_switch_latency 0 thread different
>  use_xstate = 0
>  Using threads
> 1: 100000 iters at 5174.8 ns/switch
> 2: 100000 iters at 5140.5 ns/switch
> 3: 100000 iters at 5292.9 ns/switch
>
>  ./context_switch_latency 0 process same
>  use_xstate = 0
>  Using a subprocess
> 1: 100000 iters at 2361.2 ns/switch
> 2: 100000 iters at 2332.2 ns/switch
> 3: 100000 iters at 3436.9 ns/switch
>
>  ./context_switch_latency 0 process different
>  use_xstate = 0
>  Using a subprocess
> 1: 100000 iters at 4713.6 ns/switch
> 2: 100000 iters at 4957.5 ns/switch
> 3: 100000 iters at 5012.2 ns/switch
>
>  ./context_switch_latency 1 thread same
>  use_xstate = 1
>  Using threads
> 1: 100000 iters at 2505.6 ns/switch
> 2: 100000 iters at 2483.1 ns/switch
> 3: 100000 iters at 2479.7 ns/switch
>
>  ./context_switch_latency 1 thread different
>  use_xstate = 1
>  Using threads
> 1: 100000 iters at 5245.9 ns/switch
> 2: 100000 iters at 5241.1 ns/switch
> 3: 100000 iters at 5220.3 ns/switch
>
>  ./context_switch_latency 1 process same
>  use_xstate = 1
>  Using a subprocess
> 1: 100000 iters at 2329.8 ns/switch
> 2: 100000 iters at 2350.2 ns/switch
> 3: 100000 iters at 2500.9 ns/switch
>
>  ./context_switch_latency 1 process different
>  use_xstate = 1
>  Using a subprocess
> 1: 100000 iters at 4970.7 ns/switch
> 2: 100000 iters at 5034.0 ns/switch
> 3: 100000 iters at 4991.6 ns/switch
>

I'll fiddle with that benchmark a little bit.  Maybe I can make it
suck less.  If anyone knows a good non-micro benchmark for this, let
me know.  I refuse to use dbus as my benchmark :)

FWIW, I benchmarked cr4 vs invpcid by adding a prctl and calling it in
a loop.  If Ingo's fpu benchmark thing ever lands, I'll gladly send a
patch to add TLB flushes to it.

--Andy

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