Comment 0 for bug 1840043

[Impact]
Performance degradation for read/write workloads in bcache devices, occasional system stalls

[Description]
In the latest bcache drivers, there's a sysfs attribute that calculates bucket priority statistics in /sys/fs/bcache/*/cache0/priority_stats. Querying this file has a big performance impact on tasks that run in the same CPU, and also affects read/write performance of the bcache device itself.

This is due to the way the driver calculates the stats: the bcache buckets are locked and iterated through, collecting information about each individual bucket. An array of nbucket elements is constructed and sorted afterwards, which can cause very high CPU contention in cases of larger bcache setups.

From our tests, the sorting step of the priority_stats query causes the most expressive performance reduction, as it can hinder tasks that are not even doing any bcache IO. If a task is "unlucky" to be scheduled in the same CPU as the sysfs query, its performance will be harshly reduced as both compete for CPU time. We've had users report systems stalls of up to ~6s due to this, as a result from monitoring tools that query the priority_stats periodically (e.g. Prometheus Node Exporter from [0]). These system stalls have triggered several other issues such as ceph-mon re-elections, problems in percona-cluster and general network stalls, so the impact is not isolated to bcache IO workloads.

[0] https://github.com/prometheus/node_exporter

[Test Case]
Note: As the sorting step has the most noticeable performance impact, the test case below pins a workload and the sysfs query to the same CPU. CPU contention issues still occur without any pinning, this just removes the scheduling factor of landing in different CPUs and affecting different tasks.

1) Start a read/write workload on the bcache device with e.g. fio or dd, pinned to a certain CPU:
# taskset 0x10 dd if=/dev/zero of=/dev/bcache0 bs=4k status=progress

2) Start a sysfs query loop for the priority_stats attribute pinned to the same CPU:
# for i in {1..100000}; do taskset 0x10 cat /sys/fs/bcache/*/cache0/priority_stats

3) Monitor the read/write workload for any performance impact