On Monday 14 Oct 2019 at 14:46:58 (+0100), Valentin Schneider wrote: > (Replying to the reply because for some reason my mail client never got > your reply?!) > > On 14/10/2019 14:29, Vincent Guittot wrote: > > On Mon, 14 Oct 2019 at 14:16, Quentin Perret <qperret@xxxxxxxxxx> wrote: > >> FWIW we already clear the EAS static key properly (based on the sd > >> pointer, not the static key), so this is really only for the > >> capacity-aware stuff. > >> > > Ah, right. > > >> So what happens it you have mutiple root domains ? You might skip > >> build_sched_domains() for one of them and end up not setting the static > >> key when you should no ? > >> > >> I suppose an alternative would be to play with static_branch_inc() / > >> static_branch_dec() from build_sched_domains() or something along those > >> lines. > >> > > Hmph, so I went with the concept that having the key set should mandate > having a non-NULL sd_asym_cpucapacity domain, which is why I unset it as > soon as one CPU gets attached to a NULL domain. > > Sadly as you pointed out, this doesn't work if we have another root domain > that sees asymmetry. It also kinda sounds broken to have SDs of a root > domain that does not see asymmetry (e.g. LITTLEs only) to see that key > being set. Maybe what we want is to have a key per root domain? Right, but that's not possible by definition -- static keys aren't variables. The static keys for asym CPUs and for EAS are just to optimize the case when they're disabled, but when they _are_ enabled, you have no choice but do another per-rd check. And to clarify what I tried to say before, it might be possible to 'count' the number of RDs that have SD_ASYM_CPUCAPACITY set using static_branch_inc()/dec(), like we do for the SMT static key. I remember trying to do something like that for EAS, but that was easier said than done ... :) Thanks, Quentin
