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From: Jens Axboe <axboe@kernel.dk>
To: Kees Cook <keescook@chromium.org>
Cc: Jann Horn <jannh@google.com>, io-uring <io-uring@vger.kernel.org>,
	Will Deacon <will@kernel.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>
Subject: Re: [PATCH 07/11] io_uring: use atomic_t for refcounts
Date: Tue, 10 Dec 2019 15:55:05 -0700
Message-ID: <d6ff9af3-5e72-329c-4aed-cbe6d9373235@kernel.dk> (raw)
In-Reply-To: <201912101445.CF208B717@keescook>

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@kernel.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


  reply index

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-10 15:57 [PATCHSET 0/11] io_uring improvements/fixes for 5.5-rc Jens Axboe
2019-12-10 15:57 ` [PATCH 01/11] io_uring: allow unbreakable links Jens Axboe
2019-12-10 21:10   ` Pavel Begunkov
2019-12-10 21:12     ` Jens Axboe
2019-12-10 21:28       ` Pavel Begunkov
2019-12-10 22:17         ` Jens Axboe
2019-12-10 15:57 ` [PATCH 02/11] io-wq: remove worker->wait waitqueue Jens Axboe
2019-12-10 15:57 ` [PATCH 03/11] io-wq: briefly spin for new work after finishing work Jens Axboe
2019-12-10 15:57 ` [PATCH 04/11] io_uring: sqthread should grab ctx->uring_lock for submissions Jens Axboe
2019-12-10 15:57 ` [PATCH 05/11] io_uring: deferred send/recvmsg should assign iov Jens Axboe
2019-12-10 15:57 ` [PATCH 06/11] io_uring: don't dynamically allocate poll data Jens Axboe
2019-12-10 15:57 ` [PATCH 07/11] io_uring: use atomic_t for refcounts Jens Axboe
2019-12-10 22:04   ` Jann Horn
2019-12-10 22:21     ` Jens Axboe
2019-12-10 22:46       ` Kees Cook
2019-12-10 22:55         ` Jens Axboe [this message]
2019-12-11 10:20           ` Will Deacon
2019-12-11 16:56             ` Kees Cook
2019-12-11 17:00               ` Jens Axboe
2019-12-10 15:57 ` [PATCH 08/11] io_uring: run next sqe inline if possible Jens Axboe
2019-12-10 15:57 ` [PATCH 09/11] io_uring: only hash regular files for async work execution Jens Axboe
2019-12-10 15:57 ` [PATCH 10/11] net: make socket read/write_iter() honor IOCB_NOWAIT Jens Axboe
2019-12-10 19:37   ` David Miller
2019-12-10 20:43     ` Jens Axboe
2019-12-10 15:57 ` [PATCH 11/11] io_uring: add sockets to list of files that support non-blocking issue Jens Axboe

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