linux-kernel - Re: [PATCH 1/2] perf session: Add option to copy events when queueing

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Date:	Fri, 19 Sep 2014 12:48:21 +0400
From:	Alexander Yarygin <yarygin@...ux.vnet.ibm.com>
To:	David Ahern <dsahern@...il.com>
Cc:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel@...r.kernel.org,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...hat.com>,
	Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung.kim@....com>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Stephane Eranian <eranian@...gle.com>,
	Alexander Yarygin <yarygin@...ux.vnet.ibm.com>
Subject: Re: [PATCH 1/2] perf session: Add option to copy events when queueing

David Ahern <dsahern@...il.com> writes:

> On 9/18/14, 2:21 PM, David Ahern wrote:
>> On 9/18/14, 12:53 PM, Arnaldo Carvalho de Melo wrote:
>>> If nobody objects I'll merge this patch, as it fixes problems, but I
>>> wonder if the best wouldn't be simply not calling
>>> perf_evlist__mmap_consume() till the last event there is in fact
>>> consumed... I.e. as we _really_ consume the events, we remove it from
>>> there.
>>>
>>> Instead of consuming the event at perf_tool->sample() time, we would
>>> do it at perf_tool->finished_round(), would that be feasible? Has anyone
>>> tried this?
>>
>> Hmmm... haven't tried this.  Conceptually it should work - at least
>> nothing comes to mind at the moment.
>
> Upon further review ...
>
> Alex you might want to try this first. Malloc and copy of all events
> is going to bring some serious overhead. Can avoid that if consuming
> the event in finished_round works.
>
> David

I've tried that:

--- a/tools/perf/builtin-kvm.c
+++ b/tools/perf/builtin-kvm.c
@@ -737,7 +737,6 @@ static s64 perf_kvm__mmap_read_idx(struct perf_kvm_stat *kvm, int idx,
                 * FIXME: Here we can't consume the event, as perf_session_queue_event will
                 *        point to it, and it'll get possibly overwritten by the kernel.
                 */
-               perf_evlist__mmap_consume(kvm->evlist, idx);
 
                if (err) {
                        pr_err("Failed to enqueue sample: %d\n", err);
@@ -787,6 +786,10 @@ static int perf_kvm__mmap_read(struct perf_kvm_stat *kvm)
        if (ntotal) {
                kvm->session->ordered_samples.next_flush = flush_time;
                err = kvm->tool.finished_round(&kvm->tool, NULL, kvm->session);
+
+               for (i = 0; i < kvm->evlist->nr_mmaps; i++)
+                       perf_evlist__mmap_consume(kvm->evlist, i);
+
                if (err) {
                        if (kvm->lost_events)
                                pr_info("\nLost events: %" PRIu64
                        "\n\n",

It did't work. Turned out that there is at least one event alive after
finished_round(), usually I get more - ~20. Not sure why, maybe it's
another problem which should be solved at first?


Also, I tried to follow 'perf-top' way:

  while (perf_evlist__mmap_read() != NULL) {
    perf_evlist__parse_sample();
    perf_event__process_sample();
    perf_evlist__mmap_consume();
  }

I.e. without session_queue. In this case perf won't crash, but it will
process significantly less events.

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