Re: [RFC PATCH v2 07/13] firmware: arm_scmi: Add notification dispatch and delivery

[Cristian Marussi <cristian.marussi@arm.com>]

Hi Lukasz

Thanks for your feedback !

On 21/02/2020 13:25, Lukasz Luba wrote:
> Hi Cristian,
> 
> I didn't want to jump into your discussion with Jim in other broader
> thread with this small thought, so I added a comment below.
> 
> On 2/14/20 3:35 PM, Cristian Marussi wrote:
>> Add core SCMI Notifications dispatch and delivery support logic which is
> [snip]
> 
>>   
>> @@ -840,6 +1071,11 @@ static struct scmi_notify_ops notify_ops = {
>>    */
>>   int scmi_notification_init(struct scmi_handle *handle)
>>   {
>> +	scmi_notify_wq = alloc_workqueue("scmi_notify",
>> +					 WQ_UNBOUND | WQ_FREEZABLE, 0);
> 
> I think it might limit some platforms. It depends on their workload.
> If they have some high priority workloads which rely on this mechanisms,
> they might need a RT task here. The workqueues would be scheduled in
> CFS, so it depends on workload in there (we might even see 10s ms delays
> in scheduling-up them). If we use RT we would grab the CPU from CFS.
> 
> It would be good if it is a customization option: which mechanism
> to use based on some a parameter. Then we could create:
> a) workqueue with the flags above
> b) workqueue with WQ_HIGHPRI (limited by minimum nice)
> c) kthread_create_worker() with RT/DL/FIFO sched policy
>    (with also a parameterized priority) 
> In default clients might use a) but when they want to tune their
> platform, they might change only a parameter in their scmi code,
> not maintaining a patch for the RT function out of tree.

In this series, I have not addressed configurability issues at all (as noted in the cover):
in fact I was thinking that stuff like WQ_HIGHPRI flags and per-protocol queue sizes could
be beneficial to be customizable depending on the specific platform, but I had not gone to
the extreme of thinking of adopting a dedicated RT kthread as a worker...good point...it
makes surely sense to have this configurable option to try to reduce the latency where possible.

I think it's important to give the user the possibility to configure the deferred worker
as you suggested, if the user decides to rely on Linux to handle a critical notification,
but I'd prefer queuing up this work you suggested on a different series on top of this one.
(which is starting to be a little to much voluminous...for being just the core support)

Regards

Cristian

> 
> Regards,
> Lukasz
>