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

From: Greg KH <gregkh@linuxfoundation.org>
To: Xuewei Zhang <xueweiz@google.com>
Cc: Phil Auld <pauld@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Anton Blanchard <anton@ozlabs.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org, stable@vger.kernel.org,
	trivial@kernel.org
Subject: Re: [PATCH] sched/fair: scale quota and period without losing quota/period ratio precision
Date: Fri, 4 Oct 2019 08:11:39 +0200	[thread overview]
Message-ID: <20191004061139.GA845981@kroah.com> (raw)
In-Reply-To: <20191004001243.140897-1-xueweiz@google.com>

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.

</formletter>