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Date: Tue, 10 Dec 2019 15:21:04 -0700
From: Jens Axboe <axboe@...nel.dk>
To: Jann Horn <jannh@...gle.com>
Cc: io-uring <io-uring@...r.kernel.org>, Will Deacon <will@...nel.org>,
Kees Cook <keescook@...omium.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 07/11] io_uring: use atomic_t for refcounts
On 12/10/19 3:04 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.
>> 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
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