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SubjectRe: [RFC PATCH v6 3/5] sched, tracing: reorganize fields of switch event struct
On Thu,  3 Aug 2023 04:33:50 -0400
Ze Gao <zegao2021@gmail.com> 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.

See the difference?

>
> #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 */
> ),
>

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