linux-kernel - Re: [PATCH] perf record: Add snapshot mode support for perf's regular events

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Date:	Tue, 24 Nov 2015 12:20:23 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	David Ahern <dsahern@...il.com>
Cc:	Yunlong Song <yunlong.song@...wei.com>, a.p.zijlstra@...llo.nl,
	paulus@...ba.org, mingo@...hat.com, linux-kernel@...r.kernel.org,
	wangnan0@...wei.com, namhyung@...nel.org, ast@...nel.org,
	masami.hiramatsu.pt@...achi.com, kan.liang@...el.com,
	adrian.hunter@...el.com, jolsa@...nel.org, bp@...en8.de,
	jean.pihet@...aro.org, rric@...nel.org, xiakaixu@...wei.com,
	hekuang@...wei.com
Subject: Re: [PATCH] perf record: Add snapshot mode support for perf's
 regular events

Em Tue, Nov 24, 2015 at 08:06:41AM -0700, David Ahern escreveu:
> On 11/24/15 7:00 AM, Yunlong Song wrote:
> >+static int record__write(struct record *rec, void *bf, size_t size)
> >+{
> >+	if (rec->memory.size && memory_enabled) {
> >+		if (perf_memory__write(&rec->memory, bf, size) < 0) {
> >+			pr_err("failed to write memory data, error: %m\n");
> >+			return -1;
> >+		}
> >+	} else {
> >+		if (perf_data_file__write(rec->session->file, bf, size) < 0) {
> >+			pr_err("failed to write perf data, error: %m\n");
> >+			return -1;
> >+		}
> >+		rec->bytes_written += size;
> >  	}
> >
> >-	rec->bytes_written += size;
> >  	return 0;
> >  }
> >
> >@@ -86,6 +214,8 @@ static int record__mmap_read(struct record *rec, int idx)
> >  	if (old == head)
> >  		return 0;
> >
> >+	memory_enabled = 1;
> >+
> >  	rec->samples++;
> >
> >  	size = head - old;
> >@@ -113,6 +243,7 @@ static int record__mmap_read(struct record *rec, int idx)
> >  	md->prev = old;
> >  	perf_evlist__mmap_consume(rec->evlist, idx);
> >  out:
> >+	memory_enabled = 0;
> >  	return rc;
> >  }
> >
> 
> So you are basically ignoring all samples until SIGUSR2 is received. That

No, he is not, its just that his code is difficult to follow, has to be
rewritten, but he is ignoring just PERF_RECORD_SAMPLE events, so it
will..

> means the resulting data file will have limited history of task events for

... have a complete history of task events, since PERF_RECORD_FORK, etc
are not being ignored.

No?

- Arnaldo

> example. And for other events the quantity is random as to when the mmaps
> were last scanned.
> 
> Your cover letter mentioned my code "just makes some count when the signal
> triggers perf sched, with no sample recording and has nothing to do with
> perf.data". That is not correct. If you look at the perf-daemon code I
> pointed you to it processes task events as they are received and saves the
> last N-events after time sorting (limited by memory or time). When a signal
> is received it processes the saved events and dumps them to stdout versus
> writing a perf.data file.
> 
> David
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