linux-kernel - Re: [PATCH] Perf: Correct Assumptions about Sample Timestamps in Passes

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Message-ID: <20131114083930.GC16543@gmail.com>
Date:	Thu, 14 Nov 2013 09:39:30 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Joseph Schuchart <joseph.schuchart@...dresden.de>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	thomas.ilsche@...dresden.de, linux-kernel@...r.kernel.org,
	Fr??d??ric Weisbecker <fweisbec@...il.com>,
	David Ahern <dsahern@...il.com>
Subject: Re: [PATCH] Perf: Correct Assumptions about Sample Timestamps in
 Passes


* Joseph Schuchart <joseph.schuchart@...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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