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

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Date:	Wed, 25 Feb 2015 11:27:04 -0500
From:	Jason Baron <jbaron@...mai.com>
To:	Ingo Molnar <mingo@...nel.org>
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

On 02/25/2015 02:38 AM, Ingo Molnar wrote:
> * 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 

I'm not sure there is a clear consensus on this change,
but I'm hoping that I've addressed the outstanding
concerns in this latest version.

I also think the addition of a way to do a 'wakeup policy'
here will open up other 'policies', such as taking into
account cpu affinity as you suggested. So, I think its
potentially an interesting direction for this code.

> 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?
>
Generally, Andrew and Al do more 'final' reviews here,
and a lot of others on lkml are always very helpful in
looking at this code. However, its not always clear, at
least to me, who I should pester.

Thanks,

-Jason
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