Messages in this thread | | | Subject | Re: [PATCH v2 02/10] cpufreq: provide data for frequency-invariant load-tracking support | From | Sudeep Holla <> | Date | Thu, 13 Jul 2017 13:54:49 +0100 |
| |
On 12/07/17 10:27, Viresh Kumar wrote: > On 12-07-17, 10:31, Peter Zijlstra wrote: >> So the problem with the thread is two-fold; one the one hand we like the >> scheduler to directly set frequency, but then we need to schedule a task >> to change the frequency, which will change the frequency and around we >> go. >> >> On the other hand, there's very nasty issues with PI. This thread would >> have very high priority (otherwise the SCHED_DEADLINE stuff won't work) >> but that then means this thread needs to boost the owner of the i2c >> mutex. And that then creates a massive bandwidth accounting hole. >> >> >> The advantage of using an interrupt driven state machine is that all >> those issues go away. >> >> But yes, whichever way around you turn things, its crap. But given the >> hardware its the best we can do. > > Thanks for the explanation Peter. > > IIUC, it will take more time to change the frequency eventually with > the interrupt-driven state machine as there may be multiple bottom > halves involved here, for supply, clk, etc, which would run at normal > priorities now. And those were boosted currently due to the high > priority sugov thread. And we are fine with that (from performance > point of view) ? > > Coming back to where we started from (where should we call > arch_set_freq_scale() from ?). > > I think we would still need some kind of synchronization between > cpufreq core and the cpufreq drivers to make sure we don't start > another freq change before the previous one is complete. Otherwise > the cpufreq drivers would be required to have similar support with > proper locking in place. >
Good point, but with firmware interface we are considering fro fast-switch, the firmware can override the previous request if it's not yet started. So I assume that's fine and expected ?
> And if the core is going to get notified about successful freq changes > (which it should IMHO),
Is that mandatory for even fast-switching ?
-- Regards, Sudeep
| |