On 8/15/23, Linus Torvalds [off-list ref] wrote:

On Tue, 15 Aug 2023 at 07:12, kernel test robot [off-list ref]
wrote:

quoted

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 ]

Ye man, I was puzzled myself but just figured it out and was about to respond ;)

# bpftrace -e 'kprobe:new_inode { @[kstack()] = count(); }'
Attaching 1 probe...

@[
    new_inode+1
    shmem_get_inode+137
    __shmem_file_setup+195
    shmem_zero_setup+46
    mmap_region+1937
    do_mmap+956
    vm_mmap_pgoff+224
    do_syscall_64+46
    entry_SYSCALL_64_after_hwframe+115
]: 2689570

the bench is doing this *A LOT* and this looks so fishy, for the bench
itself and the kernel doing it, but I'm not going to dig into any of
that.

quoted

     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

-- 
Mateusz Guzik <mjguzik gmail.com>