On 26/06/20 13:32, Patrick Bellasi wrote: > On Thu, Jun 25, 2020 at 17:43:51 +0200, Qais Yousef <qais.you...@arm.com> > wrote... > >> struct uclamp_rq was zeroed out entirely in assumption that in the first >> call to uclamp_rq_inc() they'd be initialized correctly in accordance to >> default settings. > > Perhaps I was not clear in my previous comment: > > https://lore.kernel.org/lkml/87sgekorfq.derkl...@matbug.net/ > > when I did say: > > Does not this means the problem is more likely with > uclamp_rq_util_with(), which should be guarded? > > I did not mean that we have to guard the calls to that function but > instead that we should just make that function aware of uclamp being > opted in or not. > >> But when next patch introduces a static key to skip >> uclamp_rq_{inc,dec}() until userspace opts in to use uclamp, schedutil >> will fail to perform any frequency changes because the >> rq->uclamp[UCLAMP_MAX].value is zeroed at init and stays as such. Which >> means all rqs are capped to 0 by default. > > The initialization you wants to do here it's needed because with the > current approach you keep calling the same uclamp_rq_util_with() and > keep doing min/max aggregations even when uclamp is not opted in. > But this means also that we have min/max aggregation _when not really > required_. > >> Fix it by making sure we do proper initialization at init without >> relying on uclamp_rq_inc() doing it later. > > My proposal was as simple as: > > ---8<--- > static __always_inline > unsigned long uclamp_rq_util_with(struct rq *rq, unsigned long util, > struct task_struct *p) > { > unsigned long min_util = READ_ONCE(rq->uclamp[UCLAMP_MIN].value); > unsigned long max_util = READ_ONCE(rq->uclamp[UCLAMP_MAX].value); > > + if (!static_branch_unlikely(&sched_uclamp_used)) > + return rt_task(p) ? uclamp_none(UCLAMP_MAX) : util > > if (p) { > min_util = max(min_util, uclamp_eff_value(p, UCLAMP_MIN)); > max_util = max(max_util, uclamp_eff_value(p, UCLAMP_MAX)); > } > > /* > * Since CPU's {min,max}_util clamps are MAX aggregated considering > * RUNNABLE tasks with _different_ clamps, we can end up with an > * inversion. Fix it now when the clamps are applied. > */ > if (unlikely(min_util >= max_util)) > return min_util; > > return clamp(util, min_util, max_util); > } > ---8<--- > > Such small change is more self-contained IMHO and does not remove > an existing optimizations like this lazy RQ's initialization at first > usage. > > Moreover, it can folded in the following patch, with all the other > static keys shortcuts.
I'd have to think some more over it, but I like this in that we wouldn't have to molest schedutil anymore.
