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. Do you have a link to the "nearly identical"? I can backport that patch and try on 5.2. -- Jens Axboe
