Re: [RFC 1/2] perf core: Add PERF_COUNT_SW_CGROUP_SWITCHES event

[Namhyung Kim]

On Thu, Dec 3, 2020 at 1:19 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> 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)

It'd be nice to avoid the __ prefix if possible.


>  {
> -       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)

Ditto.


>  {
> -       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) {

It seems task->sched_migrate is set only if the event is enabled,
then can we just check the value here?


> +               __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);

I was not clear about the RCU protection here.  Is it ok to access
the task's css_set directly?

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