Ordering / preemption of work in a workqueue preempt?

[anish singh]

On Fri, Nov 15, 2013 at 5:01 PM, Rajat Sharma <fs.rajat at gmail.com> wrote:
> Hi Rajat,
>
>
> On Fri, Nov 15, 2013 at 7:16 AM, Rajat Jain <rajatjain at juniper.net> wrote:
>>
>> Hi,
>>
>> I have a single work queue, on which I have scheduled a worker function
>> [using queue_work(wq, fn)] in interrupt context.
>>
>> I get the interrupt twice before the work queue gets a chance to run, and
>> hence the same function will get queued twice (with different private
>> context - arguments etc) which is fine and expected.
>>
>> Questions:
>>
>> 1) Is it possible that the instance that was queued by 2nd interrupt, can
>> get to run BEFORE the instance that was queued by 1st interrupt? In other
>> words, is reordering possible?
>
>
> It is unlikely as workqueue would have internal queing of tasks.
And where is the code to back that up?
>
>>
>>
>> 2) Is it possible that one running instance of the function, can get
>> preempted by second instance of the same work queue? I read through
>> http://lwn.net/Articles/511421/ and it talks about same work queue cannot
>> run on different CPU, but I have doubt about single CPU. If If I am writing
>> a worker function, does my code have to be ready that it can be preempted by
>> another instance of the same function?
What do you mean by 'one running instance of the function', did you mean
workqueue function?AFAIK same instance of the work queue can't run on different
CPU.You should write code such that it can be preempted by another
instanceof the
same workqueue but I think if you don't want that then you should call workqueue
API with proper input parameters.
Have a look at Documentation/workqueue.txt

>>
>
> Do you mean the system has just one CPU, then there would be just one worker
> thread in the workqueue which will pick up requests one by one. Do you see
> multiple threads?
>
>>
>> Please note that I understand that my worker function can preempted by
>> other processes, my doubts are related to the same workqueue.
>>
>> Thanks,
>>
>> Rajat
>>
>>
>
> Rajat
>
>>
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