linux-kernel - Re: [patch 13/13] signal/timer/event fds v9

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Message-id: <461011E0.1020607@argo.co.il>
Date:	Sun, 01 Apr 2007 23:11:12 +0300
From:	Avi Kivity <avi@...o.co.il>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Zach Brown <zach.brown@...cle.com>,
	Benjamin LaHaise <bcrl@...ck.org>
Subject: Re: [patch 13/13] signal/timer/event fds v9 - KAIO eventfd support
 example ...

Davide Libenzi wrote:

>> I think it's a bit too fine grained, and a new system call (io_bindfd()?)
>> would be easier to use.  In addition, you would move the eventfd_fget() out of
>> the submission path.
>>     
>
> IMO the cost of the eventfd_fget() (have you seen it?) is not worth adding 
> a new syscall.
>   

There's an atomic op there (and another on the way out).  Probably on a 
busy cacheline.  Still it's probably lost in the noise.

Regardless of that, I think that specifying the fd per submission is 
wrong.  It feels like a setup thing that needs to be done once.  We 
shouldn't skimp on syscalls, especially on something as important as 
unifying the async event model.

> Actually, the flags field that Linus suggested may be given an extra meaning of 
> "bind to ctx", that'd solve the problem w/out new syscalls.
>
>   

I don't see how.  It's still per submission.  You could do it on the 
first iocb, but that's just adding warts to the API.

You could add an IO_CMD_BIND_EVENTFD, but that feels wrong too, as it 
isn't really an I/O command.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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