Re: [PATCH v2] sched: make update_sd_pick_busiest return true on a busier sd

[Rik van Riel]

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On 07/28/2014 04:23 AM, Vincent Guittot wrote:
> On 25 July 2014 21:32, Rik van Riel <riel@xxxxxxxxxx> wrote:
>> Subject: sched: make update_sd_pick_busiest return true on a
>> busier sd
>> 
>> Currently update_sd_pick_busiest only identifies the busiest sd 
>> that is either overloaded, or has a group imbalance. When no sd
>> is imbalanced or overloaded, the load balancer fails to find the
>> busiest domain.
>> 
>> This breaks load balancing between domains that are not
>> overloaded, in the !SD_ASYM_PACKING case. This patch makes
>> update_sd_pick_busiest return true when the busiest sd yet is
>> encountered.
>> 
>> Behaviour for SD_ASYM_PACKING does not seem to match the
>> comment, but I have no hardware to test that so I have left the
>> behaviour of that code unchanged.
>> 
>> It is unclear what to do with the group_imb condition. Should
>> group_imb override a busier load? If so, should we fix
> 
> IMHO, group_imb should have a lower priority compared to
> overloaded group because it generates active migration whereas the
> use of overloaded group could solve the imbalance with normal
> migration Then, AFAICT, we already have a special way to compute
> imbalance when group_imb is set
> 
>> calculate_imbalance to return a sensible number when the
>> "busiest" node found has a below average load? We probably need
>> to fix calculate_imbalance anyway, to deal with an overloaded
>> group that happens to have a below average load...
>> 
>> Cc: mikey@xxxxxxxxxxx Cc: peterz@xxxxxxxxxxxxx Signed-off-by: Rik
>> van Riel <riel@xxxxxxxxxx> --- kernel/sched/fair.c | 18
>> +++++++++++++----- 1 file changed, 13 insertions(+), 5
>> deletions(-)
>> 
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index
>> 45943b2..c96044f 100644 --- a/kernel/sched/fair.c +++
>> b/kernel/sched/fair.c @@ -5949,6 +5949,11 @@ static inline void
>> update_sg_lb_stats(struct lb_env *env, 
>> sgs->group_has_free_capacity = 1; }
>> 
>> +static bool group_overloaded(struct sg_lb_stats *sgs) +{ +
>> return sgs->sum_nr_running > sgs->group_capacity_factor; +} + 
>> /** * update_sd_pick_busiest - return 1 on busiest group * @env:
>> The load balancing environment. @@ -5957,7 +5962,7 @@ static
>> inline void update_sg_lb_stats(struct lb_env *env, * @sgs:
>> sched_group statistics * * Determine if @sg is a busier group
>> than the previously selected - * busiest group. + * busiest
>> group. * * Return: %true if @sg is a busier group than the
>> previously selected * busiest group. %false otherwise. @@
>> -5967,13 +5972,17 @@ static bool update_sd_pick_busiest(struct
>> lb_env *env, struct sched_group *sg, struct sg_lb_stats *sgs) { +
>> if (group_overloaded(sgs) &&
>> !group_overloaded(&sds->busiest_stat))
> 
> The 1st time you run update_sd_pick_busiest, group_capacity_factor
> and sum_nr_running of sds->busiest_stat are uninitialized.

Good point, init_sd_lb_stats only zeroes out a few fields.

I will fix this in the next version.

>> +               return true; +
> 
> IIUC your new test sequence, you haven't solved the following use
> case:

Fixed in the next version, with Peter's idea.

- -- 
All rights reversed
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