Messages in this thread | | | From | Vincent Guittot <> | Date | Fri, 6 Mar 2020 16:06:12 +0100 | Subject | Re: [PATCH] sched: avoid scale real weight down to zero |
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On Thu, 5 Mar 2020 at 03:57, 王贇 <yun.wang@linux.alibaba.com> 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, since > A->cfs_rq.load.weight == B->se.load.weight == B->shares/nr_cpus, > so A->cfs_rq.load.weight become 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 > after scale down, although 'tg_shares' is 102400, shares of the se > which stand for group A on root cfs_rq become 2. > > While the se of D on root cfs_rq is far more bigger than 2, so it > wins the battle. > > Thus when scale_load_down() scale real weight down to 0, it's no > longer telling the real story, the caller will have the wrong > information and the calculation will be buggy. > > This patch add check in scale_load_down(), so the real weight will > be >= MIN_SHARES after scale, after applied the group C wins as > expected. > > Cc: Ben Segall <bsegall@google.com> > Cc: Vincent Guittot <vincent.guittot@linaro.org> > Suggested-by: Peter Zijlstra <peterz@infradead.org> > Signed-off-by: Michael Wang <yun.wang@linux.alibaba.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org> > --- > kernel/sched/sched.h | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index 2a0caf394dd4..75c283f22256 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -118,7 +118,13 @@ extern long calc_load_fold_active(struct rq *this_rq, long adjust); > #ifdef CONFIG_64BIT > # define NICE_0_LOAD_SHIFT (SCHED_FIXEDPOINT_SHIFT + SCHED_FIXEDPOINT_SHIFT) > # define scale_load(w) ((w) << SCHED_FIXEDPOINT_SHIFT) > -# define scale_load_down(w) ((w) >> SCHED_FIXEDPOINT_SHIFT) > +# define scale_load_down(w) \ > +({ \ > + unsigned long __w = (w); \ > + if (__w) \ > + __w = max(MIN_SHARES, __w >> SCHED_FIXEDPOINT_SHIFT); \ > + __w; \ > +}) > #else > # define NICE_0_LOAD_SHIFT (SCHED_FIXEDPOINT_SHIFT) > # define scale_load(w) (w) > -- > 2.14.4.44.g2045bb6 >
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