Re: [PATCH for-next 0/2] IB/mlx5: Use tasklet to decrease completions processing time in interrupts

[Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

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On 04/18/2016 09:21 AM, Matan Barak (External) wrote:
> On 18/04/2016 16:04, Christoph Hellwig wrote:
>> On Sun, Apr 17, 2016 at 05:08:39PM +0300, Matan Barak wrote:
>>> Hi Doug,
>>>
>>> The mlx5 driver handles completion callbacks inside interrupts.
>>> These callbacks could be lengthy and thus cause hard lockups.
>>> In order to avoid these lockups, we introduce a tasklet mechanism.
>>> The mlx5_ib driver uses this mechanism in order to defer its
>>> completion callbacks processing to the tasklet.
>>>
>>> This follows the same mechanism we implemented for mlx4 that
>>> successfully decreased the processing time in interrupts.
>>
>> Just curious:  how much of this time is spent inside the mlx5 driver,
>> and how much is spent in the callbacks from the consumers?  We've now
>> more than half done with switch the kernel ULPs to the new CQ API
>> which will always offload the callbacks to softirq or workqueue context,
>> so if we can avoid a previous offload the completions would be a lot
>> more efficient.
>>
> 
> In short, you could hit a situation where processing the completions in
> the interrupt takes longer than the rate at which they arrive (lab cases
> that use one event queue, but still).
> I agree that going to softirqs/WQs (like the rest of the offloads) is a
> good solution too - maybe even better than this one as the mechanism
> already exists, but why would it be a lot more efficient?
> 

Given the amount of the kernel converted to the new CQ API, are you guys
still looking to have this included?

-- 
Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
              GPG KeyID: 0E572FDD



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