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


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