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

["Matan Barak (External)" <matanb-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>]

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?

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