On 21-Mar 14:26, Quentin Perret wrote: > On Wednesday 21 Mar 2018 at 12:39:21 (+0000), Patrick Bellasi wrote: > > On 20-Mar 09:43, Dietmar Eggemann wrote: > > > From: Quentin Perret <[email protected]> > > > > [...] > > > > > +static unsigned long compute_energy(struct task_struct *p, int dst_cpu) > > > +{ > > > + unsigned long util, fdom_max_util; > > > + struct capacity_state *cs; > > > + unsigned long energy = 0; > > > + struct freq_domain *fdom; > > > + int cpu; > > > + > > > + for_each_freq_domain(fdom) { > > > + fdom_max_util = 0; > > > + for_each_cpu_and(cpu, &(fdom->span), cpu_online_mask) { > > > + util = cpu_util_next(cpu, p, dst_cpu); > > > > Would be nice to find a way to cache all these util and reuse them > > below... even just to ensure data consistency between the "cs" > > computation and its usage... > > So actually, what I can do is add something like > > fdom_tot_util += util; > > to this loop and compute > > energy = cs->power * fdom_tot_util / cs->cap; > > only once, instead of having the second loop to compute the energy. We don't > have to scale the util for each and every CPU since they share the same > cap state. That would save some divisions and ensure the consistency > between the selection of the cap state and the associated energy > computation. What do you think ?
Right, would say that under the hypothesis the we are in the same frequency domain (and we are because of fdom->span), that's basically doing: sum_i(P_x * U_i / C_x) => P_x / C_x * sum_i(U_i) Where (C_x, P_x) are the EM reported capacity and power for the expected frequency domain OPP. > Or maybe you were talking about consistency between several consecutive > calls to compute_energy() ? Nope, the above +1 > > > + fdom_max_util = max(util, fdom_max_util); > > > + } > > > + > > > + /* > > > + * Here we assume that the capacity states of CPUs belonging to > > > + * the same frequency domains are shared. Hence, we look at the > > > + * capacity state of the first CPU and re-use it for all. > > > + */ > > > + cpu = cpumask_first(&(fdom->span)); > > > + cs = find_cap_state(cpu, fdom_max_util); > > ^^^^ > > > > The above code could theoretically return NULL, although likely EAS is > > completely disabled if em->nb_cap_states == 0, right? > > That's right. sched_energy_present cannot be enabled with > em->nb_cap_states == 0, and compute_energy() is never called without > sched_energy_present in the proposed implementation. > > > > > If that's the case then, in the previous function, you can certainly > > avoid the initialization of *cs and maybe also add an explicit: > > > > BUG_ON(em->nb_cap_states == 0); > > > > which helps even just as "in code documentation". > > > > But, I'm not sure if maintainers like BUG_ON in scheduler code :) > > Yes, I'm not sure about the BUG_ON either :). FWIW, there are already some BUG_ON in fair.c... thus, if they can pinpoint a specific bug in case of errors, they should be acceptable ? > I agree that it would be nice to document somewhere that > compute_energy() is unsafe to call without sched_energy_present. > I can simply add a proper doc comment to this function actually. > Would that work ? Right, it's just that _maybe_ an explicit BUG_ON is improving the documentation by making more explicit the error on testing ? Thus, I would probably add both... but Peter will tell you for sure ;) -- #include <best/regards.h> Patrick Bellasi
