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

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Date:	Thu, 7 Jul 2016 12:59:41 +0800
From:	"Wangnan (F)" <wangnan0@...wei.com>
To:	Jiri Olsa <jolsa@...hat.com>
CC:	<acme@...nel.org>, <linux-kernel@...r.kernel.org>,
	<pi3orama@....com>, <lizefan@...wei.com>,
	He Kuang <hekuang@...wei.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Nilay Vaish <nilayvaish@...il.com>
Subject: Re: [PATCH v13 5/8] perf record: Read from overwritable ring buffer



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