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

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Date:	Wed, 18 Feb 2015 12:38:28 -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,
	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

On 02/18/2015 11:33 AM, Ingo Molnar wrote:
> * Jason Baron <jbaron@...mai.com> wrote:
>
>>> This has two main advantages: firstly it solves the 
>>> O(N) (micro-)problem, but it also more evenly 
>>> distributes events both between task-lists and within 
>>> epoll groups as tasks as well.
>> Its solving 2 issues - spurious wakeups, and more even 
>> loading of threads. The event distribution is more even 
>> between 'epoll groups' with this patch, however, if 
>> multiple threads are blocking on a single 'epoll group', 
>> this patch does not affect the the event distribution 
>> there. [...]
> Regarding your last point, are you sure about that?
>
> If we have say 16 epoll threads registered, and if the list 
> is static (no register/unregister activity), then the 
> wakeup pattern is in strict order of the list: threads 
> closer to the list head will be woken more frequently, in a 
> wake-once fashion. So if threads do just quick work and go 
> back to sleep quickly, then typically only the first 2-3 
> threads will get any runtime in practice - the wakeup 
> iteration never gets 'deep' into the list.
>
> With the round-robin shuffling of the list, the threads get 
> shuffled to the tail on wakeup, which distributes events 
> evenly: all 16 epoll threads will accumulate an even 
> distribution of runtime, statistically.
>
> Have I misunderstood this somehow?
>
>

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.

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(). 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.

Thanks,

-Jason
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