Re: [RFC PATCH v6 3/5] sched, tracing: reorganize fields of switch event struct

[Ze Gao]

On Thu, Aug 3, 2023 at 5:18 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Thu,  3 Aug 2023 04:33:50 -0400
> Ze Gao <zegao2021@xxxxxxxxx> wrote:
>
> > Report prioritiy and prev_state in 'short' to save some buffer
> > space. And also reorder the fields so that we take struct
> > alignment into consideration to make the record compact.
>
> If I were to write this, I would have wrote:
>
>   The prev_state field in the sched_switch event is assigned by
>   __trace_sched_switch_state(). The largest number that function will return
>   is TASK_REPORT_MAX which is just 0x100. There's no reason that the
>   prev_state field is a full 32 bits when it is using just 9 bits max. In
>   order to save space on the ring buffer, shrink the prev_state to 16 bits
>   (short).
>
>   Also, change the positions of the other fields to accommodate the short
>   value of prev_state to eliminate any holes that were created in the
>   structure.

Thanks for this elaboration! But I see Peter have comments on this, I'll wait
to see these resolved and then send my new changelog.

Thanks,
Ze

> >
> >  #ifdef CREATE_TRACE_POINTS
> > -static inline long __trace_sched_switch_state(bool preempt,
> > +static inline short __trace_sched_switch_state(bool preempt,
> >                                             unsigned int prev_state,
> >                                             struct task_struct *p)
> >  {
> >       unsigned int state;
> >
> >  #ifdef CONFIG_SCHED_DEBUG
> > -     BUG_ON(p != current);
> > +     WARN_ON_ONCE(p != current);
> >  #endif /* CONFIG_SCHED_DEBUG */
>
> The above needs to be a separate patch.
>
> -- Steve
>
> >
> >       /*
> > @@ -229,23 +229,23 @@ TRACE_EVENT(sched_switch,
> >       TP_ARGS(preempt, prev, next, prev_state),
> >
> >       TP_STRUCT__entry(
> > -             __array(        char,   prev_comm,      TASK_COMM_LEN   )
> >               __field(        pid_t,  prev_pid                        )
> > -             __field(        int,    prev_prio                       )
> > -             __field(        long,   prev_state                      )
> > -             __array(        char,   next_comm,      TASK_COMM_LEN   )
> >               __field(        pid_t,  next_pid                        )
> > -             __field(        int,    next_prio                       )
> > +             __field(        short,  prev_prio                       )
> > +             __field(        short,  next_prio                       )
> > +             __array(        char,   prev_comm,      TASK_COMM_LEN   )
> > +             __array(        char,   next_comm,      TASK_COMM_LEN   )
> > +             __field(        short,  prev_state                      )
> >       ),
> >
> >       TP_fast_assign(
> > -             memcpy(__entry->next_comm, next->comm, TASK_COMM_LEN);
> > -             __entry->prev_pid       = prev->pid;
> > -             __entry->prev_prio      = prev->prio;
> > -             __entry->prev_state     = __trace_sched_switch_state(preempt, prev_state, prev);
> > +             __entry->prev_pid               = prev->pid;
> > +             __entry->next_pid               = next->pid;
> > +             __entry->prev_prio              = (short) prev->prio;
> > +             __entry->next_prio              = (short) next->prio;
> >               memcpy(__entry->prev_comm, prev->comm, TASK_COMM_LEN);
> > -             __entry->next_pid       = next->pid;
> > -             __entry->next_prio      = next->prio;
> > +             memcpy(__entry->next_comm, next->comm, TASK_COMM_LEN);
> > +             __entry->prev_state             = __trace_sched_switch_state(preempt, prev_state, prev);
> >               /* XXX SCHED_DEADLINE */
> >       ),
> >
>