Re: [PATCH v4 1/2] sched/uclamp: Fix initialization of struct uclamp_rq

From: Valentin Schneider
Date: Fri Jun 26 2020 - 19:17:26 EST



On 26/06/20 13:32, Patrick Bellasi wrote:
> On Thu, Jun 25, 2020 at 17:43:51 +0200, Qais Yousef <qais.yousef@xxxxxxx> 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.derkling@xxxxxxxxxx/
>
> 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.