This has been reported upstream as well, with a tentative patch in

I've done some tests in fio to get some more data on this issue, with the 
following scenarios:
- "raw" test     -> fio test without any sysfs queries
- "sysfs" test   -> fio + scripted sysfs queries
- "mutex" test   -> fio + sysfs + mutex patch
- "resched" test -> fio + sysfs + cond_resched() patch

The "mutex patch" removed bucket locking from the sysfs query. This
caused the stats to be computed all wrong of course, but we weren't
interested in the stats themselves for now (just on the performance
impact of the bucket locking).

The "cond_resched()" patch was suggested upstream by Shile Zhang, and
introduces a cond_resched() call in the comparison function used for

Tests were run on a NVMe-backed bcache device with writeback enabled.
The full logs and graphs are available in the bcache-results.tar.gz

** Attachment added: "bcache-results.tar.gz"

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  bcache: Performance degradation when querying priority_stats

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