Messages in this thread Patch in this message | | | From | Morten Rasmussen <> | Subject | [PATCH 7/7] sched/fair: Set sd->should_idle_balance when misfit | Date | Thu, 15 Feb 2018 16:20:54 +0000 |
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From: Valentin Schneider <valentin.schneider@arm.com>
Idle balance is a great opportunity to pull a misfit task. However, there are scenarios where misfit tasks are present but idle balance is prevented by the should_idle_balance flag.
A good example of this is a workload of n identical tasks. Let's suppose we have a 2+2 Arm big.LITTLE system. We then spawn 4 fairly CPU-intensive tasks - for the sake of simplicity let's say they are just CPU hogs, even when running on big CPUs.
They are identical tasks, so on an SMP system they should all end at (roughly) the same time. However, in our case the LITTLE CPUs are less performing than the big CPUs, so tasks running on the LITTLEs will have a longer completion time.
This means that the big CPUs will complete their work earlier, at which point they should pull the tasks from the LITTLEs. What we want to happen is summarized as follows:
a,b,c,d are our CPU-hogging tasks _ signifies idling
LITTLE_0 | a a a a _ _ LITTLE_1 | b b b b _ _ ---------|------------- big_0 | c c c c a a big_1 | d d d d b b ^ ^ Tasks end on the big CPUs, idle balance happens and the misfit tasks are pulled straight away
This however won't happen, because currently the should_idle_balance flag is only set when there is any CPU that has more than one runnable task - which may very well not be the case here if our CPU-hogging workload is all there is to run.
As such, this commit sets the should_idle_balance flag in update_sg_lb_stats when a group is flagged as having a misfit task.
cc: Ingo Molnar <mingo@redhat.com> cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com> Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com> --- kernel/sched/fair.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 2d2302b7b584..d080a144f87f 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7871,8 +7871,10 @@ static inline void update_sg_lb_stats(struct lb_env *env, sgs->idle_cpus++; if (env->sd->flags & SD_ASYM_CPUCAPACITY && - !sgs->group_misfit_task_load && rq->misfit_task_load) + !sgs->group_misfit_task_load && rq->misfit_task_load) { sgs->group_misfit_task_load = rq->misfit_task_load; + *should_idle_balance = true; + } } /* Adjust by relative CPU capacity of the group */ -- 2.7.4
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