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Date: Tue, 3 Mar 2020 20:52:45 +0100 From: Peter Zijlstra <peterz@...radead.org> To: 王贇 <yun.wang@...ux.alibaba.com> Cc: Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, "open list:SCHEDULER" <linux-kernel@...r.kernel.org> Subject: Re: [RFC PATCH] sched: fix the nonsense shares when load of cfs_rq is too, small 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@...ux.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?
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