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

From: Ze Gao
Date: Thu Aug 03 2023 - 08:57:52 EST


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 */
> > ),
> >
>