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@google.com 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