linux-kernel - Re: [PATCH 2/2] sched/debug: add sched_update_nr

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Message-ID: <20190904174841.GW2332@hirez.programming.kicks-ass.net>
Date:   Wed, 4 Sep 2019 19:48:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Jirka Hladký <jhladky@...hat.com>,
        Jiří Vozár <jvozar@...hat.com>,
        x86@...nel.org
Subject: Re: [PATCH 2/2] sched/debug: add sched_update_nr_running tracepoint

On Wed, Sep 04, 2019 at 03:37:11PM +0100, Qais Yousef wrote:

> I managed to hook into sched_switch to get the nr_running of cfs tasks via
> eBPF.
> 
> ```
> int on_switch(struct sched_switch_args *args) {
>     struct task_struct *prev = (struct task_struct *)bpf_get_current_task();
>     struct cgroup *prev_cgroup = prev->cgroups->subsys[cpuset_cgrp_id]->cgroup;
>     const char *prev_cgroup_name = prev_cgroup->kn->name;
> 
>     if (prev_cgroup->kn->parent) {
>         bpf_trace_printk("sched_switch_ext: nr_running=%d prev_cgroup=%s\\n",
>                          prev->se.cfs_rq->nr_running,
>                          prev_cgroup_name);
>     } else {
>         bpf_trace_printk("sched_switch_ext: nr_running=%d prev_cgroup=/\\n",
>                          prev->se.cfs_rq->nr_running);
>     }
>     return 0;
> };
> ```
> 
> You can do something similar by attaching to the sched_switch tracepoint from
> a module and a create a new event to get the nr_running.
> 
> Now this is not as accurate as your proposed new tracepoint in terms where you
> sample nr_running, but should be good enough?

The above is after deactivate() and gives an up-to-date count for
decrements. Attach something to trace_sched_wakeup() to get the
increment update.