# Re: \[PATCH v3 0/3\] epoll: introduce round robin wakeup mode

- List: linux-api
- 2 messages, 2 authors
- Page: https://ratatoskr.run/linux-api/2015/02/92824/t

## Re: \[PATCH v3 0/3\] epoll: introduce round robin wakeup mode

- From: Ingo Molnar \<hidden\>
- Date: 2015-02-25 07:38:14 
- Message: https://ratatoskr.run/linux-api/2015/02/92824.md

```text

* Jason Baron [ref] 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
```

## Re: \[PATCH v3 0/3\] epoll: introduce round robin wakeup mode

- From: Jason Baron \<jbaron@akamai.com\>
- Date: 2015-02-25 16:27:04 
- Message: https://ratatoskr.run/linux-api/2015/02/92831.md

```text
On 02/25/2015 02:38 AM, Ingo Molnar wrote:
> * Jason Baron [ref] 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
```