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Date:   Fri, 17 Nov 2023 17:09:40 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     David Howells <dhowells@redhat.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        kernel test robot <oliver.sang@intel.com>,
        oe-lkp@lists.linux.dev, lkp@intel.com,
        linux-kernel@vger.kernel.org,
        Christian Brauner <brauner@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
        Christian Brauner <christian@brauner.io>,
        Matthew Wilcox <willy@infradead.org>,
        David Laight <David.Laight@aculab.com>, ying.huang@intel.com,
        feng.tang@intel.com, fengwei.yin@intel.com
Subject: Re: [linus:master] [iov_iter] c9eec08bac: vm-scalability.throughput
 -16.9% regression
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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.

https://people.kernel.org/tglx/notes-about-netiquette