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Date: Tue, 10 Dec 2019 15:55:05 -0700
From: Jens Axboe <axboe@...nel.dk>
To: Kees Cook <keescook@...omium.org>
Cc: Jann Horn <jannh@...gle.com>, io-uring <io-uring@...r.kernel.org>,
Will Deacon <will@...nel.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 07/11] io_uring: use atomic_t for refcounts
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.
--
Jens Axboe
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