From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1754376AbbBRRpk (ORCPT <rfc822;w@1wt.eu>);
	Wed, 18 Feb 2015 12:45:40 -0500
Received: from mail-wi0-f177.google.com ([209.85.212.177]:59526 "EHLO
	mail-wi0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1752335AbbBRRpi (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 18 Feb 2015 12:45:38 -0500
Date: Wed, 18 Feb 2015 18:45:33 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Jason Baron <jbaron@akamai.com>
Cc: peterz@infradead.org, mingo@redhat.com, viro@zeniv.linux.org.uk,
        akpm@linux-foundation.org, normalperson@yhbt.net,
        davidel@xmailserver.org, mtk.manpages@gmail.com,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-api@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH v2 2/2] epoll: introduce EPOLLEXCLUSIVE and
 EPOLLROUNDROBIN
Message-ID: <20150218174533.GB31566@gmail.com>
References: <cover.1424200151.git.jbaron@akamai.com>
 <7956874bfdc7403f37afe8a75e50c24221039bd2.1424200151.git.jbaron@akamai.com>
 <20150218080740.GA10199@gmail.com>
 <54E4B2D0.8020706@akamai.com>
 <20150218163300.GA28007@gmail.com>
 <54E4CE14.5010708@akamai.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <54E4CE14.5010708@akamai.com>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org


* Jason Baron <jbaron@akamai.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