Re: [PATCH net-next] net: link_watch: prevent starvation when processing linkwatch wq

[Yunsheng Lin]

On 2019/5/28 9:17, Stephen Hemminger wrote:
> On Tue, 28 May 2019 09:04:18 +0800
> Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
> 
>> On 2019/5/27 22:58, Stephen Hemminger wrote:
>>> On Mon, 27 May 2019 09:47:54 +0800
>>> Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>>>   
>>>> When user has configured a large number of virtual netdev, such
>>>> as 4K vlans, the carrier on/off operation of the real netdev
>>>> will also cause it's virtual netdev's link state to be processed
>>>> in linkwatch. Currently, the processing is done in a work queue,
>>>> which may cause worker starvation problem for other work queue.
>>>>
>>>> This patch releases the cpu when link watch worker has processed
>>>> a fixed number of netdev' link watch event, and schedule the
>>>> work queue again when there is still link watch event remaining.
>>>>
>>>> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>  
>>>
>>> Why not put link watch in its own workqueue so it is scheduled
>>> separately from the system workqueue?  
>>
>> From testing and debuging, the workqueue runs on the cpu where the
>> workqueue is schedule when using normal workqueue, even using its
>> own workqueue instead of system workqueue. So if the cpu is busy
>> processing the linkwatch event, it is not able to process other
>> workqueue' work when the workqueue is scheduled on the same cpu.
>>
>> Using unbound workqueue may solve the cpu starvation problem.
>> But the __linkwatch_run_queue is called with rtnl_lock, so if it
>> takes a lot time to process, other need to take the rtnl_lock may
>> not be able to move forward.
> 
> Agree with the starvation issue. My cocern is that large number of
> events that end up being delayed would impact things that are actually
> watching for link events (like routing daemons).

Agreed. I am not familiar with above use cases, it would be very helpful
if someone can help testing the impact of above use case.

> 
> It probably would be not accepted to do rtnl_unlock/sched_yield/rtnl_lock
> in the loop, but that is another alternative.

Yes. But seems not very efficient to do rtnl_unlock/sched_yield/rtnl_lock
for very linkwatch_do_dev.

> 
> 
> 
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