linux-kernel - Re: [PATCH] perf tools: session: avoid infinite loop

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Date:	Wed, 16 Sep 2015 17:54:54 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Mark Rutland <mark.rutland@....com>
Cc:	linux-kernel@...r.kernel.org,
	Adrian Hunter <adrian.hunter@...el.com>,
	Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] perf tools: session: avoid infinite loop

Em Wed, Sep 16, 2015 at 06:18:49PM +0100, Mark Rutland escreveu:
> This has been observed to result in an exit-time hang when counting
> rare/unschedulable events with perf record, and can be triggered
> artificially with the script below:
> 
> ----
> #!/bin/sh
> printf "REPRO: launching perf\n";
> ./perf record -e software/config=9/ sleep 1 &
> PERF_PID=$!;
> sleep 0.002;
> kill -2 $PERF_PID;
> printf "REPRO: waiting for perf (%d) to exit...\n" "$PERF_PID";
> wait $PERF_PID;
> printf "REPRO: perf exited\n";
> ----

So, I run it here, without this patch, and get:

  [root@zoo ~]# time ./repro.sh 
  REPRO: launching perf
  REPRO: waiting for perf (766) to exit...
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.015 MB perf.data ]
  REPRO: perf exited

  real	0m1.060s
  user	0m0.018s
  sys	0m0.037s
  [root@zoo ~]# perf report --stdio
  Error:
  The perf.data file has no samples!
  # To display the perf.data header info, please use
  # --header/--header-only options.
  #
  [root@zoo ~]# perf evlist -v
  software/config=9/: type: 1, size: 112, config: 0x9, { sample_period,
  sample_freq }: 4000, sample_type: IP|TID|TIME|PERIOD, disabled: 1,
  inherit: 1, mmap: 1, comm: 1, freq: 1, enable_on_exec: 1, task: 1,
  sample_id_all: 1, exclude_guest: 1, mmap2: 1, comm_exec: 1
  [root@zoo ~]# 

What am I doing wrong? Trying to reproduce this before even looking at
the patch :-)

- Arnaldo
 
> To avoid this, have __perf_session__process_events bail out early when
> the file has no data (i.e. it has no events).
> 
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Cc: Adrian Hunter <adrian.hunter@...el.com>
> Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Jiri Olsa <jolsa@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> ---
>  tools/perf/util/session.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
> index 8a4537e..fc3f7c9 100644
> --- a/tools/perf/util/session.c
> +++ b/tools/perf/util/session.c
> @@ -1580,7 +1580,10 @@ static int __perf_session__process_events(struct perf_session *session,
>  	file_offset = page_offset;
>  	head = data_offset - page_offset;
>  
> -	if (data_size && (data_offset + data_size < file_size))
> +	if (data_size == 0)
> +		goto out;
> +
> +	if (data_offset + data_size < file_size)
>  		file_size = data_offset + data_size;
>  
>  	ui_progress__init(&prog, file_size, "Processing events...");
> -- 
> 1.9.1
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