Re: [linus:master] [locking] c8afaa1b0f: stress-ng.zero.ops_per_sec 6.3% improvement

[Linus Torvalds]

On Tue, 15 Aug 2023 at 07:12, kernel test robot <oliver.sang@xxxxxxxxx> wrote:
>
> kernel test robot noticed a 6.3% improvement of stress-ng.zero.ops_per_sec on:

WTF? That's ridiculous. Why would that even test new_inode() at all?
And why would it make any difference anyway to prefetch a new inode?
The 'zero' test claims to just read /dev/zero in a loop...

[ Goes looking ]

>      39.35            -0.3       39.09        perf-profile.calltrace.cycles-pp.inode_sb_list_add.new_inode.shmem_get_inode.__shmem_file_setup.shmem_zero_setup

Ahh. It also does the mmap side, and the shared case ends up always
creating a new inode.

And while the test only tests *reading* and the mmap is read-only, the
/dev/zero file descriptor was opened for writing too, for a different
part of a test.

So even though the mapping is never written to, MAYWRITE is set, and
so the /dev/zero mapping is done as a shared memory mapping and we
can't do it as just a private one.

That's kind of stupid and looks unintentional, but whatever.

End result: that benchmark ends up being at least partly (and a fairly
noticeable part) a shmem setup benchmark, for no actual good reason.

Oh well. I certainly don't mind the removal apparently then also
helping some odd benchmark case, but I don't think this translates to
anything real. Very random.

                    Linus