[PATCH 0/1][SRU][X/B/D/E] Fix bcache performance degradation when querying priority_stats

[Heitor Alves de Siqueira]

BugLink: https://bugs.launchpad.net/bugs/1840043

[Impact]
Querying bcache's priority_stats attribute in sysfs causes severe
performance degradation for read/write workloads and occasional system
stalls

[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 > /dev/null; done

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

[Fix]
To fix CPU contention and performance impact, a cond_resched() call is
introduced in the priority_stats sort comparison.

[Regression Potential]
Regression potential is low, as the change is confined to the
priority_stats sysfs query. In cases where frequent queries to bcache
priority_stats take place (e.g. node_exporter), the impact should be
more noticeable as those could now take a bit longer to complete. A
regression due to this patch would most likely show up as a performance
degradation in bcache-focused workloads.

Shile Zhang (1):
  bcache: add cond_resched() in __bch_cache_cmp()

 drivers/md/bcache/sysfs.c | 1 +
 1 file changed, 1 insertion(+)

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2.23.0