Le Friday 07 Sep 2018 à 13:37:49 (+0200), Peter Zijlstra a écrit :
> On Fri, Sep 07, 2018 at 09:51:04AM +0200, Vincent Guittot wrote:
> > It can happen that load_balance finds a busiest group and then a busiest rq
> > but the calculated imbalance is in fact null.
>
> Cute. Does that happen often?
I have a use case with RT tasks that reproduces the problem regularly.
It happens at least when we have CPUs with different capacity either because
of heterogeous CPU or because of RT/DL reducing available capacity for cfs
I have put the call path that trigs the problem below and accroding to the
comment it seems that we can reach similar state when playing with priority.
>
> > If the calculated imbalance is null, it's useless to try to find a busiest
> > rq as no task will be migrated and we can return immediately.
> >
> > This situation can happen with heterogeneous system or smp system when RT
> > tasks are decreasing the capacity of some CPUs.
>
> Is it the result of one of those "force_balance" conditions in
> find_busiest_group() ? Should we not fix that to then return NULL
> instead?
The UC is:
We have a newly_idle load balance that is triggered when RT task becomes idle
( but I think that I have seen that with idle load balance too)
we trigs:
if (env->idle != CPU_NOT_IDLE && group_has_capacity(env, local) &&
busiest->group_no_capacity)
goto force_balance;
In calculate_imbalance we use the path
/*
* Avg load of busiest sg can be less and avg load of local sg can
* be greater than avg load across all sgs of sd because avg load
* factors in sg capacity and sgs with smaller group_type are
* skipped when updating the busiest sg:
*/
if (busiest->avg_load <= sds->avg_load ||
local->avg_load >= sds->avg_load) {
env->imbalance = 0;
return fix_small_imbalance(env, sds);
}
but fix_small_imbalance finally decides to return without modifying imbalance
like here
if (busiest->avg_load + scaled_busy_load_per_task >=
local->avg_load + (scaled_busy_load_per_task * imbn)) {
env->imbalance = busiest->load_per_task;
return;
}
Beside this patch, I'm preparing another patch in fix small imbalance to
ensure 1 task per CPU in similar situation but according to the comment above,
we can reach this situation because of tasks priority
