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Message-ID: <4198cf3d-308e-feee-91c3-2edfd1748b4c@arm.com>
Date: Wed, 8 Jul 2020 11:44:43 +0200
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
peterz@...radead.org, juri.lelli@...hat.com, rostedt@...dmis.org,
bsegall@...gle.com, mgorman@...e.de, linux-kernel@...r.kernel.org
Cc: valentin.schneider@....com
Subject: Re: [PATCH] sched/fair: handle case of task_h_load() returning 0
On 02/07/2020 16:42, 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
I guess the issue here is that 'cfs_rq->h_load' in
task_h_load() {
struct cfs_rq *cfs_rq = task_cfs_rq(p);
...
return div64_ul(p->se.avg.load_avg * cfs_rq->h_load,
cfs_rq_load_avg(cfs_rq) + 1);
}
is still ~0 (or at least pretty small) compared to se.avg.load_avg being
1024 and cfs_rq_load_avg(cfs_rq) n*1024 in these lb occurrences.
> 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@...aro.org>
> ---
> 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;
> }
>
> #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;
I assume that this is related to the LKP mail
https://lkml.kernel.org/r/20200421004749.GC26573@shao2-debian ?
I ran the test (5.8.0-rc4 w/o vs. w/ your patch) on 'Intel(R) Xeon(R)
CPU E5-2690 v2 @ 3.00GHz' (2*2*10, 40 CPUs).
I can't see the changes in the magnitude shown in the email above (they
used a 96 CPU system though).
I used only scheduler stressor mmapfork in taskgroup /A/B/C:
stress-ng --timeout 1 --times --verify --metrics-brief --sequential 40 --class scheduler --exclude (all except mmapfork)
5.8.0-rc4-custom-dieegg01-stress-ng-base
stress-ng: info: [3720] stressor bogo ops real time usr time sys time bogo ops/s bogo ops/s
stress-ng: info: [3720] (secs) (secs) (secs) (real time) (usr+sys time)
stress-ng: info: [3720] mmapfork 40 1.98 12.53 71.12 20.21 0.48
stress-ng: info: [5201] mmapfork 40 2.50 13.10 98.61 16.01 0.36
stress-ng: info: [6682] mmapfork 40 2.58 14.80 98.63 15.88 0.36
stress-ng: info: [8195] mmapfork 40 1.79 12.57 61.61 22.31 0.54
stress-ng: info: [9679] mmapfork 40 2.20 12.17 82.66 18.20 0.42
stress-ng: info: [11164] mmapfork 40 2.61 15.09 102.86 16.86 0.37
stress-ng: info: [12773] mmapfork 40 1.89 12.32 65.09 21.15 0.52
stress-ng: info: [3883] mmapfork 40 2.14 12.90 76.73 18.68 0.45
stress-ng: info: [6845] mmapfork 40 2.25 11.83 84.06 17.80 0.42
stress-ng: info: [8326] mmapfork 40 1.76 12.93 56.65 22.70 0.57
Mean: 18.98 (σ: 2.369)
5.8.0-rc4-custom-dieegg01-stress-ng
stress-ng: info: [3895] mmapfork 40 2.40 13.56 92.83 16.67 0.38
stress-ng: info: [5379] mmapfork 40 2.08 13.65 74.11 19.23 0.46
stress-ng: info: [6860] mmapfork 40 2.15 13.72 80.24 18.62 0.43
stress-ng: info: [8341] mmapfork 40 2.37 13.74 90.93 16.85 0.38
stress-ng: info: [9822] mmapfork 40 2.10 12.48 83.85 19.09 0.42
stress-ng: info: [13816] mmapfork 40 2.05 12.13 77.64 19.49 0.45
stress-ng: info: [15297] mmapfork 40 2.53 13.16 100.26 15.84 0.35
stress-ng: info: [16780] mmapfork 40 2.00 12.10 71.25 20.02 0.48
stress-ng: info: [18262] mmapfork 40 1.73 12.24 57.69 23.09 0.57
stress-ng: info: [19743] mmapfork 40 1.78 12.51 57.89 22.48 0.57
Mean: 19.14 (σ: 2.239)
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