From: Mateusz Guzik <mjguzik@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: kernel test robot <oliver.sang@intel.com>,
oe-lkp@lists.linux.dev, lkp@intel.com,
linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org,
linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
sparclinux@vger.kernel.org, linux-fsdevel@vger.kernel.org,
ying.huang@intel.com, feng.tang@intel.com, fengwei.yin@intel.com
Subject: Re: [linus:master] [locking] c8afaa1b0f: stress-ng.zero.ops_per_sec 6.3% improvement
Date: Tue, 15 Aug 2023 09:43:04 +0200 [thread overview]
Message-ID: <CAGudoHFFdFa=0y0XSEMNF4eucngxHKs7tby3rf32A-Wn1cqivQ@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=whbOEhPUL1m8Ua-+-E7kJXED4xa+duzRF-wJKR84NAPWg@mail.gmail.com>
On 8/15/23, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Tue, 15 Aug 2023 at 07:12, kernel test robot <oliver.sang@intel.com>
> 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 ]
>
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.
>> 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>
prev parent reply other threads:[~2023-08-15 7:44 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-15 7:11 [linus:master] [locking] c8afaa1b0f: stress-ng.zero.ops_per_sec 6.3% improvement kernel test robot
2023-08-15 7:33 ` Linus Torvalds
2023-08-15 7:43 ` Mateusz Guzik [this message]
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