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SubjectRe: [PATCH] Perf: Correct Assumptions about Sample Timestamps in Passes

* Joseph Schuchart <joseph.schuchart@tu-dresden.de> wrote:

> @@ -549,15 +552,24 @@ static int flush_sample_queue(struct perf_session *s,
> return 0;
> }
>
> +static inline void set_next_flush(struct perf_session *session)
> +{
> + int i;
> + u64 min_max_timestamp = session->ordered_samples.max_timestamps[0];
> + for (i = 1; i < MAX_NR_CPUS; i++) {
> + if (min_max_timestamp > session->ordered_samples.max_timestamps[i])
> + min_max_timestamp = session->ordered_samples.max_timestamps[i];
> + }
> + session->ordered_samples.next_flush = min_max_timestamp;
> +}

> static int process_finished_round(struct perf_tool *tool,
> union perf_event *event __maybe_unused,
> struct perf_session *session)
> {
> - int ret = flush_sample_queue(session, tool);
> - if (!ret)
> - session->ordered_samples.next_flush = session->ordered_samples.max_timestamp;
> -
> + int ret;
> + set_next_flush(session);
> + ret = flush_sample_queue(session, tool);

Just a quick side note, while I realize that you are
(rightfully!) concerned about correctness primarily, if that loop
over MAX_NR_CPUS executes often enough then this might hurt
performance:

perf.h:#define MAX_NR_CPUS 256

So it might be better to maintain a rolling min_max_timestamp in
this place:

+ os->max_timestamps[sample->cpu] = timestamp;

?

If done that way then AFAICS we could even eliminate the
->max_timestamps[NR_CPUS] array.

Thanks,

Ingo


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