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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, May 03, 2024 at 04:00:20PM +0200, Jesper Dangaard Brouer wrote:
> 
> 
[...]
> > 
> > I may have mistakenly thinking the lock hold time refers to just the
> > cpu_lock. Your reported times here are about the cgroup_rstat_lock.
> > Right? If so, the numbers make sense to me.
> > 
> 
> True, my reported number here are about the cgroup_rstat_lock.
> Glad to hear, we are more aligned then :-)
> 
> Given I just got some prod machines online with this patch
> cgroup_rstat_cpu_lock tracepoints, I can give you some early results,
> about hold-time for the cgroup_rstat_cpu_lock.

Oh you have already shared the preliminary data.

> 
> From this oneliner bpftrace commands:
> 
>   sudo bpftrace -e '
>          tracepoint:cgroup:cgroup_rstat_cpu_lock_contended {
>            @start[tid]=nsecs; @cnt[probe]=count()}
>          tracepoint:cgroup:cgroup_rstat_cpu_locked {
>            $now=nsecs;
>            if (args->contended) {
>              @wait_per_cpu_ns=hist($now-@start[tid]); delete(@start[tid]);}
>            @cnt[probe]=count(); @locked[tid]=$now}
>          tracepoint:cgroup:cgroup_rstat_cpu_unlock {
>            $now=nsecs;
>            @locked_per_cpu_ns=hist($now-@locked[tid]); delete(@locked[tid]);
>            @cnt[probe]=count()}
>          interval:s:1 {time("%H:%M:%S "); print(@wait_per_cpu_ns);
>            print(@locked_per_cpu_ns); print(@cnt); clear(@cnt);}'
> 
> Results from one 1 sec period:
> 
> 13:39:55 @wait_per_cpu_ns:
> [512, 1K)              3 |      |
> [1K, 2K)              12 |@      |
> [2K, 4K)             390
> |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
> [4K, 8K)              70 |@@@@@@@@@      |
> [8K, 16K)             24 |@@@      |
> [16K, 32K)           183 |@@@@@@@@@@@@@@@@@@@@@@@@      |
> [32K, 64K)            11 |@      |
> 
> @locked_per_cpu_ns:
> [256, 512)         75592 |@      |
> [512, 1K)        2537357
> |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
> [1K, 2K)          528615 |@@@@@@@@@@      |
> [2K, 4K)          168519 |@@@      |
> [4K, 8K)          162039 |@@@      |
> [8K, 16K)         100730 |@@      |
> [16K, 32K)         42276 |      |
> [32K, 64K)          1423 |      |
> [64K, 128K)           89 |      |
> 
>  @cnt[tracepoint:cgroup:cgroup_rstat_cpu_lock_contended]: 3 /sec
>  @cnt[tracepoint:cgroup:cgroup_rstat_cpu_unlock]: 3200  /sec
>  @cnt[tracepoint:cgroup:cgroup_rstat_cpu_locked]: 3200  /sec
> 
> 
> So, we see "flush-code-path" per-CPU-holding @locked_per_cpu_ns isn't
> exceeding 128 usec.

Hmm 128 usec is actually unexpectedly high. How does the cgroup
hierarchy on your system looks like? How many cgroups have actual
workloads running? Can the network softirqs run on any cpus or smaller
set of cpus? I am assuming these softirqs are processing packets from
any or all cgroups and thus have larger cgroup update tree. I wonder if
you comment out MEMCG_SOCK stat update and still see the same holding
time.

> 
> My latency requirements, to avoid RX-queue overflow, with 1024 slots,
> running at 25 Gbit/s, is 27.6 usec with small packets, and 500 usec
> (0.5ms) with MTU size packets.  This is very close to my latency
> requirements.
> 
> --Jesper
>