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Jesper Dangaard Brouer 21c38a3bd4 cgroup/rstat: add cgroup_rstat_cpu_lock helpers and tracepoints 
This closely resembles helpers added for the global cgroup_rstat_lock in
commit fc29e04ae1 ("cgroup/rstat: add cgroup_rstat_lock helpers and
tracepoints"). This is for the per CPU lock cgroup_rstat_cpu_lock.

Based on production workloads, we observe the fast-path "update" function
cgroup_rstat_updated() is invoked around 3 million times per sec, while the
"flush" function cgroup_rstat_flush_locked(), walking each possible CPU,
can see periodic spikes of 700 invocations/sec.

For this reason, the tracepoints are split into normal and fastpath
versions for this per-CPU lock. Making it feasible for production to
continuously monitor the non-fastpath tracepoint to detect lock contention
issues. The reason for monitoring is that lock disables IRQs which can
disturb e.g. softirq processing on the local CPUs involved. When the
global cgroup_rstat_lock stops disabling IRQs (e.g converted to a mutex),
this per CPU lock becomes the next bottleneck that can introduce latency
variations.

A practical bpftrace script for monitoring contention latency:

 bpftrace -e '
   tracepoint:cgroup:cgroup_rstat_cpu_lock_contended {
     @start[tid]=nsecs; @cnt[probe]=count()}
   tracepoint:cgroup:cgroup_rstat_cpu_locked {
     if (args->contended) {
       @wait_ns=hist(nsecs-@start[tid]); delete(@start[tid]);}
     @cnt[probe]=count()}
   interval:s:1 {time("%H:%M:%S "); print(@wait_ns); print(@cnt); clear(@cnt);}'

Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
2024-05-14 09:43:17 -10:00
..
cgroup-internal.h cgroup: Add a new helper for cgroup1 hierarchy 2023-11-09 13:25:47 -10:00
cgroup-v1.c cgroup: Avoid unnecessary looping in cgroup_no_v1() 2024-04-19 05:43:36 -10:00
cgroup.c cgroup: don't call cgroup1_pidlist_destroy_all() for v2 2024-04-18 05:56:58 -10:00
cpuset.c cgroup/cpuset: Remove outdated comment in sched_partition_write() 2024-04-25 07:16:19 -10:00
debug.c kernel: cgroup: fix misuse of %x 2019-05-06 08:47:48 -07:00
freezer.c cgroup: cleanup comments 2022-03-13 19:19:27 -10:00
legacy_freezer.c cgroup, legacy_freezer: update comment for freezer_css_offline() 2024-04-18 06:01:50 -10:00
Makefile cgroup: Add misc cgroup controller 2021-04-04 13:34:46 -04:00
misc.c cgroup/misc: Store atomic64_t reads to u64 2023-07-21 08:10:06 -10:00
namespace.c cgroup:namespace: Remove unused cgroup_namespaces_init() 2023-08-14 14:29:47 -10:00
pids.c cgroup/pids: Remove superfluous zeroing 2024-04-16 09:18:36 -10:00
rdma.c rdmacg: fix kernel-doc warnings in rdmacg 2023-06-05 09:45:14 -10:00
rstat.c cgroup/rstat: add cgroup_rstat_cpu_lock helpers and tracepoints 2024-05-14 09:43:17 -10:00