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

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Date:	Thu, 14 Nov 2013 11:05:52 +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:


> > 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.
> 
> I can understand your performance concerns. However, I am not 
> sure how we can determine the minimal max_timestamp of all cpus 
> without storing the information on a per-cpu basis first. 
> Accumulating it on the fly would only lead to a global 
> max_timestamp. [...]

Ok. So this:

+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;
+}

which should IMHO be written in a bit clearer form as:

static inline void set_next_flush(struct perf_session *session)
{
	u64 *timestamps = session->ordered_samples.max_timestamps;
	u64 min_timestamp = timestamps[0];
 	int i;

	for (i = 1; i < MAX_NR_CPUS; i++) {
		if (min_timestamp > timestamps[i])
			min_timestamp = timestamps[i];
	}

	session->ordered_samples.next_flush = min_timestamp;
}

calculates the minimum of the max_timestamps[] array, right?

Now, the max_timestamps[] array gets modified only in a single 
place, from the sample timestamps, via:

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

My suggestion was an identity transformation: to calculate the 
minimum of the array when the max_timestamps[] array is modified. 
A new minimum happens if the freshly written value is smaller 
than the current minimum.

I.e. the max_timestamps[] array itself is redundant, and we just 
have to update a rolling minimum - which is a (session-) global 
minimum - which is equivalent to the more complex minimum 
calculation in your patch.

What am I missing?

Thanks,

	Ingo
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