Re: [PATCH] sched/fair: scale quota and period without losing quota/period ratio precision

Greg KH
2019-10-04T06:11:45Z

On Thu, Oct 03, 2019 at 05:12:43PM -0700, Xuewei Zhang wrote:
> quota/period ratio is used to ensure a child task group won't get more
> bandwidth than the parent task group, and is calculated as:
> normalized_cfs_quota() = [(quota_us << 20) / period_us]
>
> If the quota/period ratio was changed during this scaling due to
> precision loss, it will cause inconsistency between parent and child
> task groups. See below example:
> A userspace container manager (kubelet) does three operations:
> 1) Create a parent cgroup, set quota to 1,000us and period to 10,000us.
> 2) Create a few children cgroups.
> 3) Set quota to 1,000us and period to 10,000us on a child cgroup.
>
> These operations are expected to succeed. However, if the scaling of
> 147/128 happens before step 3), quota and period of the parent cgroup
> will be changed:
> new_quota: 1148437ns, 1148us
> new_period: 11484375ns, 11484us
>
> And when step 3) comes in, the ratio of the child cgroup will be 104857,
> which will be larger than the parent cgroup ratio (104821), and will
> fail.
>
> Scaling them by a factor of 2 will fix the problem.
>
> Fixes: 2e8e19226398 ("sched/fair: Limit sched_cfs_period_timer() loop to avoid hard lockup")
> Signed-off-by: Xuewei Zhang <xueweiz@google.com>
> ---
> kernel/sched/fair.c | 36 ++++++++++++++++++++++--------------
> 1 file changed, 22 insertions(+), 14 deletions(-)
>

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree. Please read:
https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.