linux-kernel - Re: [PATCH v2 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN

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Date:	Wed, 18 Feb 2015 18:45:33 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Jason Baron <jbaron@...mai.com>
Cc:	peterz@...radead.org, mingo@...hat.com, viro@...iv.linux.org.uk,
	akpm@...ux-foundation.org, normalperson@...t.net,
	davidel@...ilserver.org, mtk.manpages@...il.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-api@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH v2 2/2] epoll: introduce EPOLLEXCLUSIVE and
 EPOLLROUNDROBIN


* Jason Baron <jbaron@...mai.com> wrote:

> So in the case of multiple threads per epoll set, we 
> currently add to the head of wakeup queue exclusively in 
> 'epoll_wait()', and then subsequently remove from the 
> queue once 'epoll_wait()' returns. So I don't think this 
> patch addresses balancing on a per epoll set basis.

Okay, so I was confused about how the code works.

> I think we could address the case you describe by simply 
> doing __add_wait_queue_tail_exclusive() instead of 
> __add_wait_queue_exclusive() in epoll_wait(). [...]

Yes.

> [...] However, I think the userspace API change is less 
> clear since epoll_wait() doesn't currently have an 
> 'input' events argument as epoll_ctl() does.

... but the change would be a bit clearer and somewhat more 
flexible: LIFO or FIFO queueing, right?

But having the queueing model as part of the epoll context 
is a legitimate approach as well.

Thanks,

	Ingo
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