On Tue, Jun 19, 2018 at 02:24:49PM +0100, Quentin Perret wrote: > On Tuesday 19 Jun 2018 at 14:26:32 (+0200), Peter Zijlstra wrote:
> > I'm confused by this patch,... what does it do? Why is em_cpu_get() > > (after you fix it) not sufficient? > > Hmm, so maybe the confusing part is that this patch does two things: > 1. it checks all conditions for starting EAS are met > (SD_ASYM_CPUCAPACITY is set, the EM covers all online CPUs, the EM > isn't too complex to be used during wakeup); > 2. it builds a list of frequency domains for the private use of the > scheduler in latency sensitive code paths, 3. and sets the static key > So I guess your question is more about 2. It is nice to have a list of > frequency domains because that makes iteration over frequency domains > in the wake-up path very easy, and efficient (for_each_freq_domain() is > used in find_energy_efficient_cpu() and compute_energy(), patches 07 and > 09/10). > And also, by making the scheduler maintain that list, we can be more > hotplug-aware. If you hotplug out all CPUs of a freq domain, the scheduler > can remove it from its list and have one less element to iterate against. > The idea was tp remove the unused things on hotplug, just like for > sched domains. > > I think that not having that list would mean to play with cpumasks in > find_energy_efficient_cpu() and in compute_energy() to keep track of the > CPUs we have visited and stuff like that. That's doable but probably more > complex, and not more efficient, I think. > > Is the overall idea any clearer ? Right, so I would not do that many things at once. Also be more explicit about what data structure, and why. That said, I think the whole for_each_freq_domain() thing as done is broken. You've completely ignored the arguments to partition_sched_domains(). What happens if you create partitions right along the frequency domains? So you really want an argument to for_each_freq_domain() to indicate who's frequency domains you want to iterate. And then I think it's easiest if you hook into build_sched_domains() instead, because you really want a list per root_domain I suspect (and an empty list if there is but one entry on).
