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SubjectRe: [RFC PATCH v5 4/7] sched: bias task wakeups to preferred semi-idle packages
* Peter Zijlstra <a.p.zijlstra@chello.nl> [2008-12-15 09:25:21]:

> On Mon, 2008-12-15 at 12:31 +0530, Balbir Singh wrote:
>
> > > kernel/sched_fair.c | 17 +++++++++++++++++
> > > 1 files changed, 17 insertions(+), 0 deletions(-)
> > >
> > > diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> > > index 98345e4..939f2a1 100644
> > > --- a/kernel/sched_fair.c
> > > +++ b/kernel/sched_fair.c
> > > @@ -1027,6 +1027,23 @@ static int wake_idle(int cpu, struct task_struct *p)
> > > cpumask_t tmp;
> > > struct sched_domain *sd;
> > > int i;
> > > + unsigned int chosen_wakeup_cpu;
> > > + int this_cpu;
> > > +
> > > + /*
> > > + * At POWERSAVINGS_BALANCE_WAKEUP level, if both this_cpu and prev_cpu
> > > + * are idle and this is not a kernel thread and this task's affinity
> > > + * allows it to be moved to preferred cpu, then just move!
> > > + */
> > > +
> > > + this_cpu = smp_processor_id();
> > > + chosen_wakeup_cpu =
> > > + cpu_rq(this_cpu)->rd->sched_mc_preferred_wakeup_cpu;
> > > +
> > > + if (sched_mc_power_savings >= POWERSAVINGS_BALANCE_WAKEUP &&
> > > + idle_cpu(cpu) && idle_cpu(this_cpu) && p->mm &&
> >
> > The p->mm check is racy, it needs to be done under task_lock(). The
> > best way to check for a kernel thread is get_task_mm(), followed by
> > put_task_mm() is the mm is not NULL. We also need to check to see if
> > the task is _hot_ on cpu. We should negate this optimization in case
> > chosen_wakeup_cpu is idle, so check for that as well.
>
> Sure its racy, but so what?
>
> The worst I can see it that we exclude a dying task from this logic,
> which isn't a problem at all, since its dying anyway.

Fair enough... except that they dying task will wake up a potentially
idle CPU and die.

>
> Also, I don't think you can grab task_lock() from under rq->lock...

I've not looked at how task_lock() nests under rq->lock. I'll look

>
> > > + cpu_isset(chosen_wakeup_cpu, p->cpus_allowed))
> > > + return chosen_wakeup_cpu;
> > >
> > > /*
> > > * If it is idle, then it is the best cpu to run this task.
> > >
> > >
> >
>
>

--
Balbir


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