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Date:   Thu, 3 Dec 2020 10:05:44 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Ian Rogers <irogers@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [RFC 1/2] perf core: Add PERF_COUNT_SW_CGROUP_SWITCHES event

Hi Peter,

On Thu, Dec 3, 2020 at 1:19 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Dec 03, 2020 at 12:02:04AM +0900, Namhyung Kim wrote:
>
> > +#ifdef CONFIG_CGROUP_PERF
> > +static inline void
> > +perf_sw_event_cgroup_switch(struct task_struct *prev, struct task_struct *next)
> > +{
> > +     struct cgroup *prev_cgrp, *next_cgrp;
> > +
> > +     rcu_read_lock();
> > +
> > +     prev_cgrp = task_css_check(prev, perf_event_cgrp_id, 1)->cgroup;
> > +     next_cgrp = task_css_check(next, perf_event_cgrp_id, 1)->cgroup;
> > +
> > +     if (prev_cgrp != next_cgrp)
> > +             perf_sw_event_sched(PERF_COUNT_SW_CGROUP_SWITCHES, 1, 0);
> > +
> > +     rcu_read_unlock();
> > +}
> > +#else
> > +static inline void perf_sw_event_cgroup_switch(struct task_struct *prev,
> > +                                            struct task_struct *next) {}
> > +#endif  /* CONFIG_CGROUP_PERF */
> > +
> >  extern struct static_key_false perf_sched_events;
> >
> >  static __always_inline bool
> > @@ -1220,6 +1241,7 @@ static inline void perf_event_task_sched_out(struct task_struct *prev,
> >                                            struct task_struct *next)
> >  {
> >       perf_sw_event_sched(PERF_COUNT_SW_CONTEXT_SWITCHES, 1, 0);
> > +     perf_sw_event_cgroup_switch(prev, next);
> >
> >       if (static_branch_unlikely(&perf_sched_events))
> >               __perf_event_task_sched_out(prev, next);
>
> Urgh.. that's horrible, try something like this.
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 9a38f579bc76..5eb284819ee5 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -1174,25 +1174,19 @@ DECLARE_PER_CPU(struct pt_regs, __perf_regs[4]);
>   * which is guaranteed by us not actually scheduling inside other swevents
>   * because those disable preemption.
>   */
> -static __always_inline void
> -perf_sw_event_sched(u32 event_id, u64 nr, u64 addr)
> +static __always_inline void __perf_sw_event_sched(u32 event_id, u64 nr, u64 addr)
>  {
> -       if (static_key_false(&perf_swevent_enabled[event_id])) {
> -               struct pt_regs *regs = this_cpu_ptr(&__perf_regs[0]);
> +       struct pt_regs *regs = this_cpu_ptr(&__perf_regs[0]);
>
> -               perf_fetch_caller_regs(regs);
> -               ___perf_sw_event(event_id, nr, regs, addr);
> -       }
> +       perf_fetch_caller_regs(regs);
> +       ___perf_sw_event(event_id, nr, regs, addr);
>  }
>
>  extern struct static_key_false perf_sched_events;
>
> -static __always_inline bool
> -perf_sw_migrate_enabled(void)
> +static __always_inline bool __perf_sw_enabled(int swevt)
>  {
> -       if (static_key_false(&perf_swevent_enabled[PERF_COUNT_SW_CPU_MIGRATIONS]))
> -               return true;
> -       return false;
> +       return static_key_false(&perf_swevent_enabled[swevt]);
>  }
>
>  static inline void perf_event_task_migrate(struct task_struct *task)
> @@ -1207,11 +1201,9 @@ static inline void perf_event_task_sched_in(struct task_struct *prev,
>         if (static_branch_unlikely(&perf_sched_events))
>                 __perf_event_task_sched_in(prev, task);
>
> -       if (perf_sw_migrate_enabled() && task->sched_migrated) {
> -               struct pt_regs *regs = this_cpu_ptr(&__perf_regs[0]);
> -
> -               perf_fetch_caller_regs(regs);
> -               ___perf_sw_event(PERF_COUNT_SW_CPU_MIGRATIONS, 1, regs, 0);
> +       if (__perf_sw_enabled(PERF_COUNT_SW_CPU_MIGRATIONS) &&
> +           task->sched_migrated) {
> +               __perf_sw_event_sched(PERF_COUNT_SW_CPU_MIGRATIONS, 1, 0);
>                 task->sched_migrated = 0;
>         }
>  }
> @@ -1219,7 +1211,13 @@ static inline void perf_event_task_sched_in(struct task_struct *prev,
>  static inline void perf_event_task_sched_out(struct task_struct *prev,
>                                              struct task_struct *next)
>  {
> -       perf_sw_event_sched(PERF_COUNT_SW_CONTEXT_SWITCHES, 1, 0);
> +       if (__perf_sw_enabled(PERF_COUNT_SW_CONTEXT_SWITCHES))
> +               __perf_sw_event_sched(PERF_COUNT_SW_CONTEXT_SWITCHES, 1, 0);
> +
> +       if (__perf_sw_enabled(PERF_COUNT_SW_CGROUP_SWITCHES) &&
> +           (task_css_check(prev, perf_event_cgrp_id, 1)->cgroup !=
> +            task_css_check(next, perf_event_cgrp_id, 1)->cgroup))
> +               __perf_sw_event_sched(PERF_COUNT_SW_CGROUP_SWITCHES, 1, 0);

Right, it needs to check only when the event is enabled.

Thanks,
Namhyung


>
>         if (static_branch_unlikely(&perf_sched_events))
>                 __perf_event_task_sched_out(prev, next);
> @@ -1475,8 +1473,6 @@ static inline int perf_event_refresh(struct perf_event *event, int refresh)
>  static inline void
>  perf_sw_event(u32 event_id, u64 nr, struct pt_regs *regs, u64 addr)    { }
>  static inline void
> -perf_sw_event_sched(u32 event_id, u64 nr, u64 addr)                    { }
> -static inline void
>  perf_bp_event(struct perf_event *event, void *data)                    { }
>
>  static inline int perf_register_guest_info_callbacks

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