Uptime related server crashes

This is a guest post by Iliya Polihronov.  Iliya is the newest member of the global infrastructure, systems, and security team at Automattic and the first ever guest blogger here on barry.wordpress.com.

Hey, my name is Iliya and as a Systems Wrangler at Automattic, I am one of the people handling the server-side issues across the 2000 servers running WordPress.com and other Automattic services.

Last week, within two hours of each other, two of our MogileFS storage servers locked up with the following trace:

The next day, a few more servers crashed with similar traces.

We started searching for a common pattern. All hosts were running Debian kernels ranging from 2.6.32-21 to 2.6.32-24, some of them were in different data centers and had different purposes in our network.

One thing we noticed was that all of the servers crashed after having an uptime of a little more than 200 days. After some research and investigation, we found that the culprit appears to be a quite interesting kernel bug.

As part of the scheduler load balancing algorithm, the kernel searches for the busiest group within a given scheduling domain. In order to do that it has to take into account the average load for all groups. It is calculated in the function find_busiest_group() with:

sds.avg_load = (SCHED_LOAD_SCALE * sds.total_load) / sds.total_pwr;

sds.total_load is the sum of the load on all CPUs in the scheduling domain, based on the run queue tasks and their priority.

SCHED_LOAD_SCALE is a constant used to increase resolution.

sds.total_pwr is the sum of the power of all CPUs in the scheduling domain. This sum ends up being zero and that’s what causing the crash – division by zero.

The “CPU power” is used to take into account how much calculating capabilities a CPU has compared to the other CPUs and the main factors for calculating it are:

1. Whether the CPU is shared, for example by using multithreading.
2. How many real-time tasks the CPU is processing.
3. In newer kernels,  how much time the CPU had spent processing IRQs.

The current suggested fix for this bug is relying on the theory that while taking into account the real-time tasks (#2 above), scale_rt_power() could return negative value, and thus the sum of all CPU powers may end up being zero.

This was merged into the vanilla kernel, together with the IRQ accounting into the cpu_power (#3  above). It is also merged into the Debian 2.6.32-31 kernel.

Alternatively, the scheduling load balancing can be turned off, which would effectively skip the related code. This can be done using control groups, however it should be used with caution as it may cause performance issues:

mount -t cgroup -o cpuset cpuset /cgroups
echo 0 > /cgroups/cpuset.sched_load_balance

As it is yet not absolutely clear if the suggested fix really fixes the problem, we will try to post updates on any new developments as we observe them.

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