Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 4 Mar 2020 09:45:44 +0100 | Subject | Re: [RFC PATCH] sched: fix the nonsense shares when load of cfs_rq is too, small |
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On Tue, 3 Mar 2020 at 20:52, Peter Zijlstra <peterz@infradead.org> wrote: > > 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?
yes and other places like the load_avg that will stay to 0 or the fact that weight != 0 is used to assume that se on enqueued and to not remove the cfs from the leaf_cfs_rq_list even if load_avg is null
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