Messages in this thread Patch in this message | | | From | bsegall@google ... | Subject | Re: [RFC PATCH] sched: fix the nonsense shares when load of cfs_rq is too, small | Date | Wed, 04 Mar 2020 10:47:12 -0800 |
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Peter Zijlstra <peterz@infradead.org> writes:
> 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?
I think instead we should probably scale_load_down(tg_shares) and scale_load(load_avg). tg_shares is always a scaled integer, so just moving the source of the scaling in the multiply should do the job.
ie
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index fcc968669aea..6d7a9d72d742 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -3179,9 +3179,9 @@ static long calc_group_shares(struct cfs_rq *cfs_rq) long tg_weight, tg_shares, load, shares; struct task_group *tg = cfs_rq->tg; - tg_shares = READ_ONCE(tg->shares); + tg_shares = scale_load_down(READ_ONCE(tg->shares)); - load = max(scale_load_down(cfs_rq->load.weight), cfs_rq->avg.load_avg); + load = max(cfs_rq->load.weight, scale_load(cfs_rq->avg.load_avg)); tg_weight = atomic_long_read(&tg->load_avg);
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