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

[Kleber Souza]

On 10.10.19 22:19, Heitor Alves de Siqueira wrote:
> 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(+)
> 

Acked-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>