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

[Frederic Weisbecker]

On Fri, Sep 06, 2013 at 01:37:01PM -0600, David Ahern wrote:
> When processing events the session code has an ordered samples queue which is
> used to time-sort events coming in across multiple mmaps. At a later point in
> time samples on the queue are flushed up to some timestamp at which point the
> event is actually processed.
> 
> When analyzing events live (ie., record/analysis path in the same command)
> there is a race that leads to corrupted events and parse errors which cause
> perf to terminate. The problem is that when the event is placed in the ordered
> samples queue it is only a reference to the event which is really sitting in
> the mmap buffer. Even though the event is queued for later processing the mmap
> tail pointer is updated which indicates to the kernel that the event has been
> processed. The race is flushing the event from the queue before it gets
> overwritten by some other event. For commands trying to process events live
> (versus just writing to a file) and processing a high rate of events this leads
> to parse failures and perf terminates.
> 
> Examples hitting this problem are 'perf kvm stat live', especially with nested
> VMs which generate 100,000+ traces per second, and a command processing
> scheduling events with a high rate of context switching -- e.g., running
> 'perf bench sched pipe'.
> 
> This patch offers live commands an option to copy the event when it is placed in
> the ordered samples queue.
> 
> Signed-off-by: David Ahern <dsahern@xxxxxxxxx>
> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: Mike Galbraith <efault@xxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Stephane Eranian <eranian@xxxxxxxxxx>
> ---
>  tools/perf/util/session.c |   17 ++++++++++++++---
>  tools/perf/util/session.h |    1 +
>  2 files changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
> index 1b185ca..71f16db 100644
> --- a/tools/perf/util/session.c
> +++ b/tools/perf/util/session.c
> @@ -483,6 +483,8 @@ static void perf_session_free_sample_buffers(struct perf_session *session)
>  
>  		sq = list_entry(os->to_free.next, struct sample_queue, list);
>  		list_del(&sq->list);
> +		if (session->copy_on_queue)
> +			free(sq->event);
>  		free(sq);
>  	}
>  }
> @@ -513,11 +515,15 @@ static int flush_sample_queue(struct perf_session *s,
>  			break;
>  
>  		ret = perf_evlist__parse_sample(s->evlist, iter->event, &sample);
> -		if (ret)
> +		if (ret) {
>  			pr_err("Can't parse sample, err = %d\n", ret);
> -		else {
> +			if (s->copy_on_queue)
> +				free(iter->event);
> +		} else {
>  			ret = perf_session_deliver_event(s, iter->event, &sample, tool,
>  							 iter->file_offset);
> +			if (s->copy_on_queue)
> +				free(iter->event);
>  			if (ret)
>  				return ret;
>  		}
> @@ -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;
>  
>  	__queue_event(new, s);
>  
> diff --git a/tools/perf/util/session.h b/tools/perf/util/session.h
> index 3aa75fb..4adfcbb 100644
> --- a/tools/perf/util/session.h
> +++ b/tools/perf/util/session.h
> @@ -38,6 +38,7 @@ struct perf_session {
>  	bool			fd_pipe;
>  	bool			repipe;
>  	struct ordered_samples	ordered_samples;
> +	bool			copy_on_queue;

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?

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"?

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.
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