Messages in this thread Patch in this message |  | | From | Valentin Schneider <> | Subject | Re: [PATCH] sched/fair: handle case of task_h_load() returning 0 | Date | Thu, 09 Jul 2020 14:06:35 +0100 |
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On 02/07/20 15:42, Vincent Guittot wrote: > task_h_load() can return 0 in some situations like running stress-ng > mmapfork, which forks thousands of threads, in a sched group on a 224 cores > system. The load balance doesn't handle this correctly because > env->imbalance never decreases and it will stop pulling tasks only after > reaching loop_max, which can be equal to the number of running tasks of > the cfs. Make sure that imbalance will be decreased by at least 1. > > misfit task is the other feature that doesn't handle correctly such > situation although it's probably more difficult to face the problem > because of the smaller number of CPUs and running tasks on heterogenous > system. > > We can't simply ensure that task_h_load() returns at least one because it > would imply to handle underrun in other places. > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
I dug some more into this; if I got my math right, this can be reproduced with a single task group below the root. Forked tasks get max load, so this can be tried out with either tons of forks or tons of CPU hogs.
We need
p->se.avg.load_avg * cfs_rq->h_load ----------------------------------- < 1 cfs_rq_load_avg(cfs_rq) + 1
Assuming homogeneous system with tasks spread out all over (no other tasks interfering), that should boil down to
1024 * (tg.shares / nr_cpus) --------------------------- < 1 1024 * (nr_tasks_on_cpu)
IOW
tg.shares / nr_cpus < nr_tasks_on_cpu
If we get tasks nicely spread out, a simple condition to hit this should be to have more tasks than shares.
I can hit task_h_load=0 with the following on my Juno (pinned to one CPU to make things simpler; big.LITTLE doesn't yield equal weights between CPUs):
cgcreate -g cpu:tg0
echo 128 > /sys/fs/cgroup/cpu/tg0/cpu.shares
for ((i=0; i<130; i++)); do # busy loop of your choice taskset -c 0 ./loop.sh & echo $! > /sys/fs/cgroup/cpu/tg0/tasks done
> --- > kernel/sched/fair.c | 18 +++++++++++++++++- > 1 file changed, 17 insertions(+), 1 deletion(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 6fab1d17c575..62747c24aa9e 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -4049,7 +4049,13 @@ static inline void update_misfit_status(struct task_struct *p, struct rq *rq) > return; > } > > - rq->misfit_task_load = task_h_load(p); > + /* > + * Make sure that misfit_task_load will not be null even if > + * task_h_load() returns 0. misfit_task_load is only used to select > + * rq with highest load so adding 1 will not modify the result > + * of the comparison. > + */ > + rq->misfit_task_load = task_h_load(p) + 1;
For here and below; wouldn't it be a tad cleaner to just do
foo = max(task_h_load(p), 1);
Otherwise, I think I've properly convinced myself we do want to have that in one form or another. So either way:
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
> } > > #else /* CONFIG_SMP */ > @@ -7664,6 +7670,16 @@ static int detach_tasks(struct lb_env *env) > env->sd->nr_balance_failed <= env->sd->cache_nice_tries) > goto next; > > + /* > + * Depending of the number of CPUs and tasks and the > + * cgroup hierarchy, task_h_load() can return a null > + * value. Make sure that env->imbalance decreases > + * otherwise detach_tasks() will stop only after > + * detaching up to loop_max tasks. > + */ > + if (!load) > + load = 1; > + > env->imbalance -= load; > break;
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