Re: [RFC PATCH 4/7] sched: cpufreq: Remove magic 1.25 headroom from apply_dvfs_headroom()

Qais Yousef
2023-09-10T19:23:31Z

On 09/07/23 13:34, Peter Zijlstra wrote:
> On Mon, Aug 28, 2023 at 12:32:00AM +0100, Qais Yousef wrote:
> > Instead of the magical 1.25 headroom, use the new approximate_util_avg()
> > to provide headroom based on the dvfs_update_delay; which is the period
> > at which the cpufreq governor will send DVFS updates to the hardware.
> >
> > Add a new percpu dvfs_update_delay that can be cheaply accessed whenever
> > apply_dvfs_headroom() is called. We expect cpufreq governors that rely
> > on util to drive its DVFS logic/algorithm to populate these percpu
> > variables. schedutil is the only such governor at the moment.
> >
> > Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> > ---
> > kernel/sched/core.c | 3 ++-
> > kernel/sched/cpufreq_schedutil.c | 10 +++++++++-
> > kernel/sched/sched.h | 25 ++++++++++++++-----------
> > 3 files changed, 25 insertions(+), 13 deletions(-)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 602e369753a3..f56eb44745a8 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -116,6 +116,7 @@ EXPORT_TRACEPOINT_SYMBOL_GPL(sched_util_est_se_tp);
> > EXPORT_TRACEPOINT_SYMBOL_GPL(sched_update_nr_running_tp);
> >
> > DEFINE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
> > +DEFINE_PER_CPU_SHARED_ALIGNED(u64, dvfs_update_delay);
>
> This makes no sense, why are you using SHARED_ALIGNED and thus wasting
> an entire cacheline for the one variable?

Err, brain fart. Sorry.

Thanks

--
Qais Yousef