Re: [RFC PATCH v4 3/7] sched: nominate preferred wakeup cpu

From: Vaidyanathan Srinivasan
Date: Mon Nov 24 2008 - 01:10:59 EST


* Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> [2008-11-23 03:03:58]:

> On Fri, 2008-11-21 at 14:01 +0530, Vaidyanathan Srinivasan wrote:
> > When the system utilisation is low and more cpus are idle,
> > then the process waking up from sleep should prefer to
> > wakeup an idle cpu from semi-idle cpu package (multi core
> > package) rather than a completely idle cpu package which
> > would waste power.
> >
> > Use the sched_mc balance logic in find_busiest_group() to
> > nominate a preferred wakeup cpu.
> >
> > This info can be sored in appropriate sched_domain, but
> > updating this info in all copies of sched_domain is not
> > practical. Hence this information is stored in root_domain
> > struct which is one copy per partitioned sched domain.
> > The root_domain can be accessed from each cpu's runqueue
> > and there is one copy per partitioned sched domain.
> >
> > Signed-off-by: Vaidyanathan Srinivasan <svaidy@xxxxxxxxxxxxxxxxxx>
> > ---
> >
> > kernel/sched.c | 15 +++++++++++++++
> > 1 files changed, 15 insertions(+), 0 deletions(-)
> >
> > diff --git a/kernel/sched.c b/kernel/sched.c
> > index 79b71f3..d28cd98 100644
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -493,6 +493,17 @@ struct root_domain {
> > #ifdef CONFIG_SMP
> > struct cpupri cpupri;
> > #endif
> > +#if defined(CONFIG_SCHED_MC) || defined(CONFIG_SCHED_SMT)
> > +
> > + /*
> > + * Preferred wake up cpu nominated by sched_mc balance that will be
> > + * used when most cpus are idle in the system indicating overall very
> > + * low system utilisation. Triggered at POWERSAVINGS_BALANCE_WAKEUP(2)
> > + */
> > + unsigned int sched_mc_preferred_wakeup_cpu;
> > +
> > +#endif
> > +
> > };
> >
> > /*
>
> Do we really need that extra whitespace?
>
> > @@ -3090,6 +3101,7 @@ static int move_one_task(struct rq *this_rq, int this_cpu, struct rq *busiest,
> > return 0;
> > }
> >
> > +
> > /*
> > * find_busiest_group finds and returns the busiest CPU group within the
> > * domain. It calculates and returns the amount of weighted load which
>
> Ditto?
>
> > @@ -3406,6 +3418,9 @@ out_balanced:
> >
> > if (this == group_leader && group_leader != group_min) {
> > *imbalance = min_load_per_task;
> > + if (sched_mc_power_savings >= POWERSAVINGS_BALANCE_WAKEUP)
> > + cpu_rq(this_cpu)->rd->sched_mc_preferred_wakeup_cpu =
> > + first_cpu(group_leader->cpumask);
>
> While not strictly needed, I prefer braces around multi-line statments.
> It's easier on the eyes.

Hi Peter,

Thanks for the detailed review. I will fix these issues in the next
iteration.

--Vaidy
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