linux-kernel - Re: [RFC PATCH 02/10] sched: Task placement for heterogeneous systems based on task load-tracking

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Date:	Tue, 9 Oct 2012 22:28:21 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Morten Rasmussen <Morten.Rasmussen@....com>
Cc:	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"pjt@...gle.com" <pjt@...gle.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"suresh.b.siddha@...el.com" <suresh.b.siddha@...el.com>,
	"linaro-sched-sig@...ts.linaro.org" 
	<linaro-sched-sig@...ts.linaro.org>,
	"linaro-dev@...ts.linaro.org" <linaro-dev@...ts.linaro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Amit Kucheria <amit.kucheria@...aro.org>,
	Robin Randhawa <Robin.Randhawa@....com>,
	Arvind Chauhan <Arvind.Chauhan@....com>
Subject: Re: [RFC PATCH 02/10] sched: Task placement for heterogeneous systems
 based on task load-tracking

On 9 October 2012 21:26, Morten Rasmussen <Morten.Rasmussen@....com> wrote:
> On Thu, Oct 04, 2012 at 07:02:03AM +0100, Viresh Kumar wrote:
>> On 22 September 2012 00:02,  <morten.rasmussen@....com> wrote:

>> > SCHED_HMP requires that the platform implements arch_get_hmp_domains()
>> > which should set up the platform specific list of hmp_domains. It is
>> > also assumed that the platform disables SD_LOAD_BALANCE for the
>> > appropriate sched_domains.
>>
>> An explanation of this requirement would be helpful here.
>
> Yes. This is to prevent the load-balancer from moving tasks between
> hmp_domains. This will be done exclusively by SCHED_HMP instead to
> implement a strict task migration policy and avoid changing the
> load-balancer behaviour. The load-balancer will take care of
> load-balacing within each hmp_domain.

Honestly speaking i understood this point now and earlier it wasn't clear
to me :)

What would be ideal is to put this information in the comment just before
we re-define other SCHED_*** domains where we disable balancing.
And keep it in the commit log too.

>> > +struct hmp_domain {
>> > +       struct cpumask cpus;
>> > +       struct list_head hmp_domains;
>>
>> Probably need a better name here. domain_list?
>
> Yes. hmp_domain_list would be better and stick with the hmp_* naming
> convention.

IMHO hmp_ would be better for global names, but names of variables
enclosed within another hmp_*** data type don't actually need hmp_**,
as this is implicity.

i.e.
struct hmp_domain {
       struct cpumask cpus;
       struct list_head domain_list;
}

would be better than
       struct list_head hmp domain_list;

as the parent structure already have hmp_***. So whatever is inside the
struct is obviously hmp specific.

>> > +/* Setup hmp_domains */
>> > +static int __init hmp_cpu_mask_setup(void)
>>
>> How should we interpret its return value? Can you mention what does 0 & 1 mean
>> here?
>>
>
> Returns 0 if domain setup failed, i.e. the domain list is empty, and 1
> otherwise.

Helpful. Please mention this in function comment in your next revision.

>> > +{
>> > +       char buf[64];
>> > +       struct hmp_domain *domain;
>> > +       struct list_head *pos;
>> > +       int dc, cpu;

>> > +       /* Print hmp_domains */
>> > +       dc = 0;
>>
>> Should be done during definition of dc.

You missed this ??

>> > +               for_each_cpu_mask(cpu, domain->cpus) {
>> > +                       per_cpu(hmp_cpu_domain, cpu) = domain;
>> > +               }
>>
>> Should use hmp_cpu_domain(cpu) here. Also no need of {} for single
>> line loop.

??

>> > +               dc++;
>>
>> You aren't using it... Only for testing? Should we remove it from mainline
>> patchset and keep it locally?
>>
>
> I'm using it in the pr_debug line a little earlier. It is used for
> enumerating the hmp_domains.

My mistake :(

>> > +/* Check if cpu is in fastest hmp_domain */
>> > +static inline unsigned int hmp_cpu_is_fastest(int cpu)
>> > +{
>> > +       struct list_head *pos;
>> > +
>> > +       pos = &hmp_cpu_domain(cpu)->hmp_domains;
>> > +       return pos == hmp_domains.next;
>>
>> better create list_is_first() for this.
>
> I had the same thought, but I see that as a separate patch that should
> be submitted separately.

Correct. So better send it now, so that it is included before you send your
next version. :)

--
viresh
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