On Wed, Dec 11, 2019 at 10:20:13AM +0000, Will Deacon wrote: > On Tue, Dec 10, 2019 at 03:55:05PM -0700, Jens Axboe wrote: > > On 12/10/19 3:46 PM, Kees Cook wrote: > > > On Tue, Dec 10, 2019 at 03:21:04PM -0700, Jens Axboe wrote: > > >> On 12/10/19 3:04 PM, Jann Horn wrote: > > >>> [context preserved for additional CCs] > > >>> > > >>> On Tue, Dec 10, 2019 at 4:57 PM Jens Axboe <axboe@xxxxxxxxx> wrote: > > >>>> Recently had a regression that turned out to be because > > >>>> CONFIG_REFCOUNT_FULL was set. > > >>> > > >>> I assume "regression" here refers to a performance regression? Do you > > >>> have more concrete numbers on this? Is one of the refcounting calls > > >>> particularly problematic compared to the others? > > >> > > >> Yes, a performance regression. io_uring is using io-wq now, which does > > >> an extra get/put on the work item to make it safe against async cancel. > > >> That get/put translates into a refcount_inc and refcount_dec per work > > >> item, and meant that we went from 0.5% refcount CPU in the test case to > > >> 1.5%. That's a pretty substantial increase. > > >> > > >>> I really don't like it when raw atomic_t is used for refcounting > > >>> purposes - not only because that gets rid of the overflow checks, but > > >>> also because it is less clear semantically. > > >> > > >> Not a huge fan either, but... It's hard to give up 1% of extra CPU. You > > >> could argue I could just turn off REFCOUNT_FULL, and I could. Maybe > > >> that's what I should do. But I'd prefer to just drop the refcount on the > > >> io_uring side and keep it on for other potential useful cases. > > > > > > There is no CONFIG_REFCOUNT_FULL any more. Will Deacon's version came > > > out as nearly identical to the x86 asm version. Can you share the > > > workload where you saw this? We really don't want to regression refcount > > > protections, especially in the face of new APIs. > > > > > > Will, do you have a moment to dig into this? > > > > Ah, hopefully it'll work out ok, then. The patch came from testing the > > full backport on 5.2. Oh good! I thought we had some kind of impossible workload. :) > > Do you have a link to the "nearly identical"? I can backport that > > patch and try on 5.2. > > You could try my refcount/full branch, which is what ended up getting merged > during the merge window: > > https://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git/log/?h=refcount/full Yeah, as you can see in the measured tight-loop timings in https://git.kernel.org/linus/dcb786493f3e48da3272b710028d42ec608cfda1 there was 0.1% difference for Will's series compared to the x86 assembly version, where as the old FULL was almost 70%. -- Kees Cook
