On 10/23/2013 09:30 AM, Preeti U Murthy wrote:
> Hi Peter,
>
> On 10/23/2013 03:41 AM, Peter Zijlstra wrote:
>> On Mon, Oct 21, 2013 at 05:14:42PM +0530, Vaidyanathan Srinivasan wrote:
>>> kernel/sched/fair.c | 19 +++++++++++++------
>>> 1 file changed, 13 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index 7c70201..12f0eab 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -5807,12 +5807,19 @@ static inline int nohz_kick_needed(struct rq *rq,
>>> int cpu)
>>>
>>> rcu_read_lock();
>>> for_each_domain(cpu, sd) {
>>> + struct sched_domain *sd_parent = sd->parent;
>>> + struct sched_group *sg;
>>> + struct sched_group_power *sgp;
>>> + int nr_busy;
>>> +
>>> + if (sd_parent) {
>>> + sg = sd_parent->groups;
>>> + sgp = sg->sgp;
>>> + nr_busy = atomic_read(&sgp->nr_busy_cpus);
>>> +
>>> + if (sd->flags & SD_SHARE_PKG_RESOURCES && nr_busy > 1)
>>> + goto need_kick_unlock;
>>> + }
>>>
>>> if (sd->flags & SD_ASYM_PACKING && nr_busy != sg->group_weight
>>> && (cpumask_first_and(nohz.idle_cpus_mask,
>>>
>>
>> Almost I'd say; what happens on !sd_parent && SD_ASYM_PACKING ?
>
> You are right, sorry about this. The idea was to correct the nr_busy
> computation before the patch that would remove its usage in the second
> patch. But that would mean the condition nr_busy != sg->group_weight
> would be invalid with this patch. The second patch needs to go first to
> avoid this confusion.
>
>>
>> Also, this made me look at the nr_busy stuff again, and somehow that
>> entire thing makes me a little sad.
>>
>> Can't we do something like the below and cut that nr_busy sd iteration
>> short?
>
> We can surely cut the nr_busy sd iteration but not like what is done
> with this patch. You stop the nr_busy computation at the sched domain
> that has the flag SD_SHARE_PKG_RESOURCES set. But nohz_kick_needed()
> would want to know the nr_busy for one level above this.
> Consider a core. Assume it is the highest domain with this flag set.
> The nr_busy of its groups, which are logical threads are set to 1/0
> each. But nohz_kick_needed() would like to know the sum of the nr_busy
> parameter of all the groups, i.e. the threads in a core before it
> decides if it can kick nohz_idle balancing. The information about the
> individual group's nr_busy is of no relevance here.
>
> Thats why the above patch tries to get the
> sd->parent->groups->sgp->nr_busy_cpus. This will translate rightly to
> the core's busy cpus in this example. But the below patch stops before
> updating this parameter at the sd->parent level, where sd is the highest
> level sched domain with the SD_SHARE_PKG_RESOURCES flag set.
>
> But we can get around all this confusion if we can move the nr_busy
> parameter to be included in the sched_domain structure rather than the
> sched_groups_power structure. Anyway the only place where nr_busy is
> used, that is at nohz_kick_needed(), is done to know the total number of
> busy cpus at a sched domain level which has the SD_SHARE_PKG_RESOURCES
> set and not at a sched group level.
>
> So why not move nr_busy to struct sched_domain and having the below
> patch which just updates this parameter for the sched domain, sd_busy ?
Oh this can't be done :( Domain structures are per cpu!
Regards
Preeti U Murthy
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