Re: [RFC PATCH] sched: fix the nonsense shares when load of cfs_rq is too, small

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Mar 03, 2020 at 10:17:03PM +0800, 王贇 wrote:
> During our testing, we found a case that shares no longer
> working correctly, the cgroup topology is like:
> 
>   /sys/fs/cgroup/cpu/A		(shares=102400)
>   /sys/fs/cgroup/cpu/A/B	(shares=2)
>   /sys/fs/cgroup/cpu/A/B/C	(shares=1024)
> 
>   /sys/fs/cgroup/cpu/D		(shares=1024)
>   /sys/fs/cgroup/cpu/D/E	(shares=1024)
>   /sys/fs/cgroup/cpu/D/E/F	(shares=1024)
> 
> The same benchmark is running in group C & F, no other tasks are
> running, the benchmark is capable to consumed all the CPUs.
> 
> We suppose the group C will win more CPU resources since it could
> enjoy all the shares of group A, but it's F who wins much more.
> 
> The reason is because we have group B with shares as 2, which make
> the group A 'cfs_rq->load.weight' very small.
> 
> And in calc_group_shares() we calculate shares as:
> 
>   load = max(scale_load_down(cfs_rq->load.weight), cfs_rq->avg.load_avg);
>   shares = (tg_shares * load) / tg_weight;
> 
> Since the 'cfs_rq->load.weight' is too small, the load become 0
> in here, although 'tg_shares' is 102400, shares of the se which
> stand for group A on root cfs_rq become 2.

Argh, because A->cfs_rq.load.weight is B->se.load.weight which is
B->shares/nr_cpus.

> While the se of D on root cfs_rq is far more bigger than 2, so it
> wins the battle.
> 
> This patch add a check on the zero load and make it as MIN_SHARES
> to fix the nonsense shares, after applied the group C wins as
> expected.
> 
> Signed-off-by: Michael Wang <yun.wang@linux.alibaba.com>
> ---
>  kernel/sched/fair.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 84594f8aeaf8..53d705f75fa4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3182,6 +3182,8 @@ static long calc_group_shares(struct cfs_rq *cfs_rq)
>  	tg_shares = READ_ONCE(tg->shares);
> 
>  	load = max(scale_load_down(cfs_rq->load.weight), cfs_rq->avg.load_avg);
> +	if (!load && cfs_rq->load.weight)
> +		load = MIN_SHARES;
> 
>  	tg_weight = atomic_long_read(&tg->load_avg);

Yeah, I suppose that'll do. Hurmph, wants a comment though.

But that has me looking at other users of scale_load_down(), and doesn't
at least update_tg_cfs_load() suffer the same problem?