linux-kernel - Re: [PATCH v3 0/3] epoll: introduce round robin wakeup mode

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Date:	Wed, 25 Feb 2015 08:38:14 +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,
	luto@...capital.net, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alexander Viro <viro@....linux.org.uk>
Subject: Re: [PATCH v3 0/3] epoll: introduce round robin wakeup mode


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

> Hi,
> 
> When we are sharing a wakeup source among multiple epoll 
> fds, we end up with thundering herd wakeups, since there 
> is currently no way to add to the wakeup source 
> exclusively. This series introduces a new EPOLL_ROTATE 
> flag to allow for round robin exclusive wakeups.
> 
> I believe this patch series addresses the two main 
> concerns that were raised in prior postings. Namely, that 
> it affected code (and potentially performance) of the 
> core kernel wakeup functions, even in cases where it was 
> not strictly needed, and that it could lead to wakeup 
> starvation (since we were are no longer waking up all 
> waiters). It does so by adding an extra layer of 
> indirection, whereby waiters are attached to a 'psuedo' 
> epoll fd, which in turn is attached directly to the 
> wakeup source.

>   sched/wait: add __wake_up_rotate()

>  include/linux/wait.h           |  1 +
>  kernel/sched/wait.c            | 27 ++++++++++++++++++++++

So the scheduler bits are looking good to me in principle, 
because they just add a new round-robin-rotating wakeup 
variant and don't disturb the others.

Is there consensus on the epoll ABI changes? With Davide 
Libenzi inactive eventpoll appears to be without a 
dedicated maintainer since 2011 or so. Is there anyone who 
knows the code and its usages in detail and does final ABI 
decisions on eventpoll - Andrew, Al or Linus?

Thanks,

	Ingo
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