On Mon, Jun 15, 2020 at 09:32:41AM +0200, Jann Horn wrote: > On Thu, Jun 11, 2020 at 10:24 PM Linus Torvalds > <torva...@linux-foundation.org> wrote: > > > > On Wed, Jun 10, 2020 at 9:05 PM kernel test robot <rong.a.c...@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>
Yes, functionally this is the same file as what is used by 0day, though 0day use a stable release tar ball. Thanks, Feng > 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... > _______________________________________________ > LKP mailing list -- l...@lists.01.org > To unsubscribe send an email to lkp-le...@lists.01.org