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:

quoted

Add core SCMI Notifications dispatch and delivery support logic which is

[snip]

quoted

@@ -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

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