Re: [PATCH v1] cgroup/rstat: add cgroup_rstat_cpu_lock helpers and tracepoints

[Tejun Heo <tj@kernel.org>]

On Wed, May 01, 2024 at 04:04:11PM +0200, Jesper Dangaard Brouer wrote:
> This closely resembles helpers added for the global cgroup_rstat_lock in
> commit fc29e04ae1ad ("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>

Applied to cgroup/for-6.10.

Thanks.

-- 
tejun