Hi Morten, On Fri, Mar 23, 2018 at 8:47 AM, Morten Rasmussen <morten.rasmus...@arm.com> wrote: > On Thu, Mar 22, 2018 at 01:10:22PM -0700, Joel Fernandes wrote: >> On Wed, Mar 21, 2018 at 8:35 AM, Patrick Bellasi >> <patrick.bell...@arm.com> wrote: >> > [...] >> > >> >> @@ -6555,6 +6613,14 @@ select_task_rq_fair(struct task_struct *p, int >> >> prev_cpu, int sd_flag, int wake_f >> >> break; >> >> } >> >> >> >> + /* >> >> + * Energy-aware task placement is performed on the highest >> >> + * non-overutilized domain spanning over cpu and prev_cpu. >> >> + */ >> >> + if (want_energy && !sd_overutilized(tmp) && >> >> + cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) >> >> + energy_sd = tmp; >> >> + >> > >> > Not entirely sure, but I was trying to understand if we can avoid to >> > modify the definition of want_affine (in the previous chunk) and move >> > this block before the previous "if (want_affine..." (in mainline but >> > not in this chunk), which will became an else, e.g. >> > >> > if (want_energy && !sd_overutilized(tmp) && >> > // ... >> > else if (want_energy && !sd_overutilized(tmp) && >> > // ... >> > >> > Isn't that the same? >> > >> > Maybe there is a code path I'm missing... but otherwise it seems a >> > more self contained modification of select_task_rq_fair... >> >> Just replying to this here Patrick instead of the other thread. >> >> I think this is the right place for the block from Quentin quoted >> above because we want to search for the highest domain that is >> !overutilized and look among those for the candidates. So from that >> perspective, we can't move the block to the beginning and it seems to >> be in the right place. My main concern on the other thread was >> different, I was talking about the cases where sd_flag & tmp->flags >> don't match. In that case, sd = NULL would trump EAS and I was >> wondering if that's the right thing to do... > > You mean if SD_BALANCE_WAKE isn't set on sched_domains?
Yes. > The current code seems to rely on that flag to be set to work correctly. > Otherwise, the loop might bail out on !want_affine and we end up doing > the find_energy_efficient_cpu() on the lowest level sched_domain even if > there is higher level one which isn't over-utilized. > > However, SD_BALANCE_WAKE should be set if SD_ASYM_CPUCAPACITY is set so > sd == NULL shouldn't be possible? This only holds as long as we only > want EAS for asymmetric systems. Yes, I see you had topology code that set SD_BALANCE_WAKE for ASYM. It makes sense to me then, thanks for the clarification. Still I feel it is a bit tedious/confusing when reading code to draw the conclusion about why sd is checked first before doing find_energy_efficient_cpu (and that sd will != NULL for ASYM systems). If energy_sd is set, then we can just proceed with EAS without checking that sd != NULL. This function in mainline is already pretty confusing as it is :-( Regards, - Joel
