Messages in this thread | | | From | Jann Horn <> | Date | Mon, 15 Jun 2020 09:32:41 +0200 | Subject | Re: [gup] 17839856fd: stress-ng.vm-splice.ops_per_sec 2158.6% improvement |
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On Thu, Jun 11, 2020 at 10:24 PM Linus Torvalds <torvalds@linux-foundation.org> wrote: > > On Wed, Jun 10, 2020 at 9:05 PM kernel test robot <rong.a.chen@intel.com> wrote: > > > > FYI, we noticed a 2158.6% improvement of stress-ng.vm-splice.ops_per_sec due to commit: > > > > commit: 17839856fd588f4ab6b789f482ed3ffd7c403e1f ("gup: document and work around "COW can break either way" issue") > > Well, that is amusing, and seeing improvements is always nice, but > somehow I think the test is broken. > > I can't see why you'd ever see an improvement from that commit, and if > you do see one, not one by a factor of 20x.
FWIW, if this is the testcase: <https://kernel.ubuntu.com/git/cking/stress-ng.git/tree/stress-vm-splice.c>
then that testcase is essentially testing how fast vmsplice() is when called in a loop on an uninitialized mmap() mapping. So before that commit, I think it will create zeropage PTEs in the first iteration (and zeropage PTEs are _PAGE_SPECIAL, see do_anonymous_page()). And get_user_pages_fast() bails out in gup_pte_range() if pte_special(). So that testcase was always hitting the GUP slowpath. But now the first iteration will force the creation of a normal RW PTE, so all following iterations can go through the GUP fastpath.
So in summary I guess the test was just really slow up until now because it was hitting a slowpath that you wouldn't hit during normal usage? At least for vmsplice(), writing uninitialized pages doesn't really make a whole lot of sense...
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