Re: [PATCH v13 5/8] perf record: Read from overwritable ring buffer

[Wangnan (F)]

On 2016/7/6 20:34, Jiri Olsa wrote:
> On Wed, Jul 06, 2016 at 08:03:28PM +0800, Wangnan (F) wrote:
> > On 2016/7/6 19:38, Jiri Olsa wrote:
> > > On Mon, Jul 04, 2016 at 06:20:06AM +0000, Wang Nan wrote:
> > >
> > > SNIP
> > >
> > > > +static void
> > > > +record__toggle_overwrite_evsels(struct record *rec,
> > > > +				enum overwrite_evt_state state)
> > > > +{
> > > > +	struct perf_evlist *evlist = rec->overwrite_evlist;
> > > > +	enum overwrite_evt_state old_state = rec->overwrite_evt_state;
> > > > +	enum action {
> > > > +		NONE,
> > > > +		PAUSE,
> > > > +		RESUME,
> > > > +	} action = NONE;
> > > > +
> > > > +	switch (old_state) {
> > > > +	case OVERWRITE_EVT_RUNNING: {
> > > > +		switch (state) {
> > > > +		case OVERWRITE_EVT_DATA_PENDING:
> > > > +			action = PAUSE;
> > > > +			break;
> > > > +		case OVERWRITE_EVT_RUNNING:
> > > > +		case OVERWRITE_EVT_EMPTY:
> > > > +		default:
> > > > +			goto state_err;
> > > > +		}
> > > > +		break;
> > > > +	}
> > > > +	case OVERWRITE_EVT_DATA_PENDING: {
> > > > +		switch (state) {
> > > > +		case OVERWRITE_EVT_EMPTY:
> > > > +			break;
> > > > +		case OVERWRITE_EVT_RUNNING:
> > > > +		case OVERWRITE_EVT_DATA_PENDING:
> > > > +		default:
> > > > +			goto state_err;
> > > > +		}
> > > > +		break;
> > > > +	}
> > > > +	case OVERWRITE_EVT_EMPTY: {
> > > > +		switch (state) {
> > > > +		case OVERWRITE_EVT_RUNNING:
> > > > +			action = RESUME;
> > > > +			break;
> > > > +		case OVERWRITE_EVT_EMPTY:
> > > > +		case OVERWRITE_EVT_DATA_PENDING:
> > > > +		default:
> > > > +			goto state_err;
> > > > +		}
> > > > +		break;
> > > > +	}
> > > > +	default:
> > > > +		WARN_ONCE(1, "Shouldn't get there\n");
> > > > +	}
> > > > +
> > > > +	rec->overwrite_evt_state = state;
> > > > +
> > > > +	if (!evlist)
> > > > +		return;
> > > I'd expect this check at the begining
> > I think even evlist is NULL the state changing is still required.
> > Actually, the state machine is independent with aux evlist. Even
> > we without overwritable evsels the state machine is still valid.
> > So let the state machine runs unconditionally.
> hum, can't see that.. it's state machine to govern overwrite evlist, right?
> if there's no overwrite evlist we should keep the current processing

Not as easy as I thought. Look at following code:

> @@ -1006,8 +1122,27 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
> 		}
>
> 		if (trigger_is_hit(&switch_output_trigger)) {
> +			/*
> +			 * If switch_output_trigger is hit, the data in
> +			 * overwritable ring buffer should have been collected,
> +			 * so overwrite_evt_state should be set to
> +			 * OVERWRITE_EVT_EMPTY.
> +			 *
> +			 * If SIGUSR2 raise after or during record__mmap_read_all(),
> +			 * record__mmap_read_all() didn't collect data from
> +			 * overwritable ring buffer. Read again.
> +			 */
> +			if (rec->overwrite_evt_state == OVERWRITE_EVT_RUNNING)
> +				continue;
> 			trigger_ready(&switch_output_trigger);
>
> +			/*
> +			 * Reenable events in overwrite ring buffer after
> +			 * record__mmap_read_all(): we should have collected
> +			 * data from it.
> +			 */
> +			record__toggle_overwrite_evsels(rec, OVERWRITE_EVT_RUNNING);
> +
> 			if (!quiet)
> 				fprintf(stderr, "[ perf record: dump data: Woken up %ld times ]\n",
> 					waking);

Here perf tests whether reading from overwritable ring buffer is required.
If SIGUSR2 is received just before the above trigger_is_hit, we should 
read from
overwrite ring buffer again. The OVERWRITE_EVT_RUNNING checker is for 
this reason.

Now if we stop the state machine, the state is stopped at 
OVERWRITE_EVT_RUNNING,
causes perf loops forever.

We can check rec->overwrite_evlist first, but it is ugly, since I 
believe the
overwritable state is independent to overwrite evlist. So I decide to 
introduce
a new state indicate the overwrite evlist is not ready.

Thank you.


> if it's meant to govern the mmap reading in general
> we should at least rename it
> jirka