[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20191211102012.GA4123@willie-the-truck>
Date: Wed, 11 Dec 2019 10:20:13 +0000
From: Will Deacon <will@...nel.org>
To: Jens Axboe <axboe@...nel.dk>
Cc: Kees Cook <keescook@...omium.org>, Jann Horn <jannh@...gle.com>,
io-uring <io-uring@...r.kernel.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 07/11] io_uring: use atomic_t for refcounts
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@...nel.dk> 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.
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
Will
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
