Thanks Steven,
Indeed this did not consider the struct alignment. I left the field order intact
because I was not sure whether the order here matters for reporting or parsing.
Now I will fix this and send a v3, and again thanks for pointing it out.
Thanks,
Ze
On Mon, Jul 31, 2023 at 11:38 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> 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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