On Sun, Nov 13, 2016 at 8:47 PM, Steve Muckle smuckle.linux@gmail.com wrote:
On Sun, Nov 13, 2016 at 03:37:18PM +0100, Rafael J. Wysocki wrote:
Hold on a sec. I thought during LPC someone (Peter?) made a point that when RT thread run, we should bump the frequency to max? So, schedutil is going to trigger schedutil to bump up the frequency to max, right?
No, it isn't, or at least that is unlikely.
sugov_update_commit() sets sg_policy->work_in_progress before queuing the IRQ work and it is not cleared until the frequency changes in sugov_work().
OTOH, sugov_should_update_freq() checks sg_policy->work_in_progress upfront and returns false when it is set, so the governor won't see its own worker threads run, unless I'm overlooking something highly non-obvious.
FWIW my intention with the original version of this patch (which I neglected to communicate to Viresh) was that it would depend on changing the frequency policy for RT. I had been using rt_avg. It sounds like during LPC there were talks of using another metric.
It does appear things would work okay without that but it also seems a bit fragile.
Yes, it does.
To a minimum, there should be a comment regarding that in the patches.
There's the window between when the work_in_progress gets cleared and the RT kthread yields. I have not thought through the various scenarios there, what is possible and tested to see if it is significant enough to impact power-sensitive platforms.
Well, me neither, to be entirely honest. :-)
That said, there is a limited number of call sites for update_curr_rt(), where SCHED_CPUFREQ_RT is passed to cpufreq governors: dequeue_task_rt(), put_prev_task_rt(), pick_next_task_rt(), and task_tick_rt(). I'm not sure how pick_next_task_rt() can be relevant here at all, though, and task_tick_rt() would need to be running exactly during the window mentioned above, so it probably is negligible either, at least on the average.
From the quick look at the scheduler core, put_prev_task() is mostly
called for running tasks, so that case doesn't look like something to worry about too, although it would need to be looked through in detail. The dequeue part I'm totally unsure about.
In any case, the clearing of work_in_progress might still be deferred by queuing a regular (non-RT) work item to do that from the kthread work (that will guarantee "hiding" the kthread work from the governor), but admittedly that would be a sledgehammer of sorts (and it might defeat the purpose of the whole exercise) ...
Thanks, Rafael