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From: Jens Axboe <axboe@kernel.dk>
To: Jann Horn <jannh@google.com>
Cc: io-uring <io-uring@vger.kernel.org>,
	Will Deacon <will@kernel.org>, Kees Cook <keescook@chromium.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 15:21:04 -0700
Message-ID: <02ba41a9-14f2-e3be-f43f-99f311c662ef@kernel.dk> (raw)
In-Reply-To: <CAG48ez3yh7zRhMyM+VhH1g9Gp81_3FMjwAyj3TB6HQYETpxHmA@mail.gmail.com>

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.

>> Our ref count usage is really simple,
> 
> In my opinion, for a refcount to qualify as "really simple", it must
> be possible to annotate each relevant struct member and local variable
> with the (fixed) bias it carries when alive and non-NULL. This
> refcount is more complicated than that.

:-(

-- 
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 [this message]
2019-12-10 22:46       ` Kees Cook
2019-12-10 22:55         ` Jens Axboe
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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