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

[Lukasz Luba]

On 2/21/20 7:11 PM, Lukasz Luba wrote:
> On 2/21/20 7:01 PM, Cristian Marussi wrote:
> > 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 = {
> > > > ÂÂÂ */
> > > > ÂÂ int scmi_notification_init(struct scmi_handle *handle)
> > > > ÂÂ {
> > > > +ÂÂÂ scmi_notify_wq = alloc_workqueue("scmi_notify",
> > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 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
> > > ÂÂÂ (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)
>
> Agree, you can build these features incrementally.

Although, a WQ_SYSFS flag wouldn't harm too much this version and might
give possibility to tune/experiment with it.

> Regards,
> Lukasz