Re: [PATCH 3/3] rseq/selftests: Add support for arm64

Will Deacon
2018-06-28T16:46:26Z

Hi Mathieu,

On Tue, Jun 26, 2018 at 12:11:52PM -0400, Mathieu Desnoyers wrote:
> ----- On Jun 26, 2018, at 11:14 AM, Will Deacon will.deacon@arm.com wrote:
> > On Mon, Jun 25, 2018 at 02:10:10PM -0400, Mathieu Desnoyers wrote:
> >> I notice you are using the instructions
> >>
> >> adrp
> >> add
> >> str
> >>
> >> to implement RSEQ_ASM_STORE_RSEQ_CS(). Did you compare
> >> performance-wise with an approach using a literal pool
> >> near the instruction pointer like I did on arm32 ?
> >
> > I didn't, no. Do you have a benchmark to hand so I can give this a go?
>
> see tools/testing/selftests/rseq/param_test_benchmark --help
>
> It's a stripped-down version of param_test, without all the code for
> delay loops and testing checks.
>
> Example use for counter increment with 4 threads, doing 5G counter
> increments per thread:
>
> time ./param_test_benchmark -T i -t 4 -r 5000000000

Thanks. I ran that on a few arm64 systems I have access to, with three
configurations of the selftest:

1. As I posted
2. With the abort signature and branch in-lined, so as to avoid the CBNZ
address limitations in large codebases
3. With both the abort handler and the table inlined (i.e. the same thing
as 32-bit).

There isn't a reliably measurable difference between (1) and (2), but I take
between 12% and 27% hit between (2) and (3).

So I'll post a v2 based on (2).

Will