Re: [PATCH] perf session: Add option to copy events when queueing

From: David Ahern
Date: Sat Sep 14 2013 - 13:25:52 EST


On 9/14/13 10:16 AM, Frederic Weisbecker wrote:
@@ -676,7 +682,12 @@ int perf_session_queue_event(struct perf_session *s, union perf_event *event,

new->timestamp = timestamp;
new->file_offset = file_offset;
- new->event = event;
+
+ if (s->copy_on_queue) {
+ new->event = malloc(event->header.size);
+ memcpy(new->event, event, event->header.size);
+ } else
+ new->event = event;

---8<---

So do you think it should stay optional? This looks like a global problem, I mean
the event can be unmapped anytime for any builtin tool mapping it, right?

Yes. I could make it the default behavior; just overhead in doing that (malloc/copy for each event).


Also we already allocate the sample list node (struct sample_queue) from os->sample
buffer. ie: we have our own allocator there.

Probably we should reuse that and include the copied event space in "struct sample_queue"?


Right, that's where I put the malloc and copy - I kept the relevant change above. I take it you are thinking of something different but I am not following you. You definitely do NOT want to change struct sample_queue to include an event - like this:

diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
index 51f5edf..866944a 100644
--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -491,7 +491,7 @@ static perf_event__swap_op perf_event__swap_ops[] = {
struct sample_queue {
u64 timestamp;
u64 file_offset;
- union perf_event *event;
+ union perf_event event;
struct list_head list;
};

size of event is determined by mmap_event (mmap2_event in latest code) which is > 4096 because of the filename argument. Including the event directly in sample_queue would balloon memory usage (learned this the hard way!).


Also looking at it now, it seems we have a bug on the existing code:


if (!list_empty(sc)) {
new = list_entry(sc->next, struct sample_queue, list);
list_del(&new->list);
} else if (os->sample_buffer) {
new = os->sample_buffer + os->sample_buffer_idx;
if (++os->sample_buffer_idx == MAX_SAMPLE_BUFFER)
os->sample_buffer = NULL;
} else {
os->sample_buffer = malloc(MAX_SAMPLE_BUFFER * sizeof(*new));
if (!os->sample_buffer)
return -ENOMEM;
list_add(&os->sample_buffer->list, &os->to_free);
os->sample_buffer_idx = 2;
new = os->sample_buffer + 1;
}

If we actually run out of buffer rooms, we should realloc right after and not
wait for the next entry, otherwise we loose an event:

if (!list_empty(sc)) {
new = list_entry(sc->next, struct sample_queue, list);
list_del(&new->list);
} else {
if (os->sample_buffer) {
new = os->sample_buffer + os->sample_buffer_idx;
if (++os->sample_buffer_idx == MAX_SAMPLE_BUFFER)
os->sample_buffer = NULL;
}

if (!os->sample_buffer) {
os->sample_buffer = malloc(MAX_SAMPLE_BUFFER * sizeof(*new));
if (!os->sample_buffer)
return -ENOMEM;
list_add(&os->sample_buffer->list, &os->to_free);
os->sample_buffer_idx = 2;
new = os->sample_buffer + 1;
}


Although the mirrored os->sample_buffer condition check is a bit ugly and should move to
a function. But the idea is there.

Ok. That should be a separate patch. Are you going to submit that one?

David

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