Re: [PATCH] sched/fair: Fix bandwidth timer clock drift condition

[Greg KH <greg@kroah.com>]

On Wed, Jan 16, 2019 at 07:52:08PM +0000, Alakesh Haloi wrote:
> [ Upstream commit 512ac999d2755d2b7109e996a76b6fb8b888631d ]
> 
> I noticed that cgroup task groups constantly get throttled even
> if they have low CPU usage, this causes some jitters on the response
> time to some of our business containers when enabling CPU quotas.
> 
> It's very simple to reproduce:
> 
>   mkdir /sys/fs/cgroup/cpu/test
>   cd /sys/fs/cgroup/cpu/test
>   echo 100000 > cpu.cfs_quota_us
>   echo $$ > tasks
> 
> then repeat:
> 
>   cat cpu.stat | grep nr_throttled  # nr_throttled will increase steadily
> 
> After some analysis, we found that cfs_rq::runtime_remaining will
> be cleared by expire_cfs_rq_runtime() due to two equal but stale
> "cfs_{b|q}->runtime_expires" after period timer is re-armed.
> 
> The current condition to judge clock drift in expire_cfs_rq_runtime()
> is wrong, the two runtime_expires are actually the same when clock
> drift happens, so this condtion can never hit. The orginal design was
> correctly done by this commit:
> 
>   a9cf55b28610 ("sched: Expire invalid runtime")
> 
> ... but was changed to be the current implementation due to its locking bug.
> 
> This patch introduces another way, it adds a new field in both structures
> cfs_rq and cfs_bandwidth to record the expiration update sequence, and
> uses them to figure out if clock drift happens (true if they are equal).
> 
> Signed-off-by: Xunlei Pang <xlpang@linux.alibaba.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> [alakeshh: backport: Fixed merge conflicts:
>  - sched.h: Fix the indentation and order in which the variables are
>    declared to match with coding style of the existing code in 4.14
>    Struct members of same type were declared in separate lines in
>    upstream patch which has been changed back to having multiple
>    members of same type in the same line.
>    e.g. int a; int b; ->  int a, b; ]

Now queued up, thanks!

greg k-h