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

[Peter Zijlstra]

On Mon, Oct 07, 2019 at 11:14:25AM -0400, Phil Auld wrote:
> 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@xxxxxxxxxx>
> 
> 
> I managed to get it to trigger the second case. It took 50,000 children (20x my initial tests).
> 
> [ 1367.850630] cfs_period_timer[cpu11]: period too short, scaling up (new cfs_period_us = 4340, cfs_quota_us = 250000)
> [ 1370.390832] cfs_period_timer[cpu11]: period too short, scaling up (new cfs_period_us = 8680, cfs_quota_us = 500000)
> [ 1372.914689] cfs_period_timer[cpu11]: period too short, scaling up (new cfs_period_us = 17360, cfs_quota_us = 1000000)
> [ 1375.447431] cfs_period_timer[cpu11]: period too short, scaling up (new cfs_period_us = 34720, cfs_quota_us = 2000000)
> [ 1377.982785] cfs_period_timer[cpu11]: period too short, scaling up (new cfs_period_us = 69440, cfs_quota_us = 4000000)
> [ 1380.481702] cfs_period_timer[cpu11]: period too short, scaling up (new cfs_period_us = 138880, cfs_quota_us = 8000000)
> [ 1382.894692] cfs_period_timer[cpu11]: period too short, scaling up (new cfs_period_us = 277760, cfs_quota_us = 16000000)
> [ 1385.264872] cfs_period_timer[cpu11]: period too short, scaling up (new cfs_period_us = 555520, cfs_quota_us = 32000000)
> [ 1393.965140] cfs_period_timer[cpu11]: period too short, but cannot scale up without losing precision (cfs_period_us = 555520, cfs_quota_us = 32000000)
> 
> I suspect going higher could cause the original lockup, but that'd be the case with the old code as well. 
> And this also gets us out of it faster.
> 
> 
> Tested-by: Phil Auld <pauld@xxxxxxxxxx>

Thanks!