Re: [linus:master] [iov_iter] c9eec08bac: vm-scalability.throughput -16.9% regression

Borislav Petkov
2023-11-17T16:10:12Z

On Fri, Nov 17, 2023 at 02:12:17PM +0000, David Howells wrote:
> Borislav Petkov <bp@alien8.de> wrote:
>
> > Hmm, stupid question: are those results better than without that
> > oneliner?
>
> Without any changes, I see something like:
>
> iov_kunit_benchmark_bvec: avg 3185 uS, stddev 19 uS
> iov_kunit_benchmark_bvec: avg 3186 uS, stddev 9 uS
> iov_kunit_benchmark_bvec: avg 3244 uS, stddev 153 uS
> iov_kunit_benchmark_bvec_split: avg 3397 uS, stddev 16 uS
> iov_kunit_benchmark_bvec_split: avg 3400 uS, stddev 16 uS
> iov_kunit_benchmark_bvec_split: avg 3402 uS, stddev 34 uS
> iov_kunit_benchmark_kvec: avg 2818 uS, stddev 550 uS
> iov_kunit_benchmark_kvec: avg 2906 uS, stddev 21 uS
> iov_kunit_benchmark_kvec: avg 2923 uS, stddev 1496 uS
> iov_kunit_benchmark_xarray: avg 3564 uS, stddev 6 uS
> iov_kunit_benchmark_xarray: avg 3573 uS, stddev 17 uS
> iov_kunit_benchmark_xarray: avg 3575 uS, stddev 58 uS
> iov_kunit_benchmark_xarray_to_bvec: avg 3929 uS, stddev 9 uS
> iov_kunit_benchmark_xarray_to_bvec: avg 3930 uS, stddev 6 uS
> iov_kunit_benchmark_xarray_to_bvec: avg 3930 uS, stddev 7 uS

Which looks like those added memcpy calls add a lot of overhead due to
the mitigations crap.

You could verify that if you boot a kernel with the oneliner but add
"mitigations=off" on the cmdline.

Or profile the workload and check whether the return thunks appear
higher in the profile... I'd say.

Thx.

--
Regards/Gruss,
Boris.