linux-kernel - Re: [RFC PATCH v5 4/7] sched: bias task wakeups to preferred semi-idle packages

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Date:	Mon, 15 Dec 2008 09:33:04 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	balbir@...ux.vnet.ibm.com
Cc:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Suresh B Siddha <suresh.b.siddha@...el.com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	Ingo Molnar <mingo@...e.hu>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Vatsa <vatsa@...ux.vnet.ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Andi Kleen <andi@...stfloor.org>,
	David Collier-Brown <davecb@....com>,
	Tim Connors <tconnors@...ro.swin.edu.au>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Gregory Haskins <gregory.haskins@...il.com>
Subject: Re: [RFC PATCH v5 4/7] sched: bias task wakeups to preferred
 semi-idle packages

On Mon, 2008-12-15 at 09:25 +0100, Peter Zijlstra wrote:
> 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.

At which point I seriously doubt it'd still be on the rq anyway.

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