On Wed, 26 Jul 2023 20:16:16 +0800
Ze Gao <zegao2021@xxxxxxxxx> wrote:
> @@ -231,41 +253,29 @@ TRACE_EVENT(sched_switch,
> TP_STRUCT__entry(
> __array( char, prev_comm, TASK_COMM_LEN )
> __field( pid_t, prev_pid )
> - __field( int, prev_prio )
> - __field( long, prev_state )
> + __field( short, prev_prio )
> + __field( int, prev_state )
> + __field( char, prev_state_char )
> __array( char, next_comm, TASK_COMM_LEN )
> __field( pid_t, next_pid )
> - __field( int, next_prio )
> + __field( short, next_prio )
> ),
The above adds a bunch of holes. This needs to be reordered to condense the
event, we don't want to increase it. libtraceevent will handle reordering.
The above produces:
struct {
char prev_comm[16];
pid_t prev_pid;
short prev_prio; <-- 2 character padding
int prev_state;
char prev_state_char;
char next_comm[16]; <- 3 character padding
pid_t next_pid;
short next_prio; <- 2 char padding
};
(all events are at least 4 byte aligned, and are multiple of 4 bytes in
size, thus that last short of next_prio did nothing)
The above is a total of 56 bytes (note, that is the same as the current
sched_switch event size);
What the above should be:
TP_STRUCT__entry(
__field( pid_t, prev_pid )
__field( pid_t, next_pid )
__field( short, prev_prio )
__field( short, next_prio )
__field( int, prev_state )
__array( char, prev_comm, TASK_COMM_LEN )
__array( char, next_comm, TASK_COMM_LEN )
__field( char, prev_state_char )
),
Which would be:
struct {
pid_t prev_pid;
pid_t next_pid;
short prev_prio;
short next_prio;
int prev_state;
char prev_comm[16];
char next_comm[16];
char prev_stat_char; <-- 3 characters of padding
}
which would be 52 byte. Saving us 4 bytes per event. Which is a big deal!
-- Steve
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