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

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Date:	Fri, 18 Sep 2015 10:51:13 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] perf tools: session: avoid infinite loop

On Fri, Sep 18, 2015 at 07:09:16AM +0100, Adrian Hunter wrote:
> On 17/09/15 18:41, Mark Rutland wrote:
> > Hi,
> > 
> > On Wed, Sep 16, 2015 at 09:54:54PM +0100, Arnaldo Carvalho de Melo wrote:
> >> 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
> > 
> > [...]
> > 
> >> What am I doing wrong? Trying to reproduce this before even looking at
> >> the patch :-)
> > 
> > I suspect you have a shinier computer than I do! ;)
> > 
> 
> I imagine you would also need to contrive not to write any synthesized MMAP
> or COMM events i.e. no access to /proc perhaps?

Taking a look through __cmd_record:

I'm not writing to a pipe, so nothing gets synthesized for attrs or tracepoints.

I'm not using aux trace, so nothing gets synthesized for that.

Running as a regular user I don't have synthesized events for the kernel mmap
or modules:

	WARNING: Kernel address maps (/proc/{kallsyms,modules}) are restricted,
	check /proc/sys/kernel/kptr_restrict.

	Samples in kernel functions may not be resolved if a suitable vmlinux
	file is not found in the buildid cache or in the vmlinux path.

	Samples in kernel modules won't be resolved at all.

	If some relocation was applied (e.g. kexec) symbols may be misresolved
	even with a suitable vmlinux or kallsyms file.

	Cannot read kernel map
	Couldn't record kernel reference relocation symbol
	Symbol resolution may be skewed if relocation was used (e.g. kexec).
	Check /proc/kallsyms permission or run as root.

I'm not running a guest, so I don't have any synthesized events for the guest
os.

__machine__synthesize_threads doesn't synthesize any task events
(target__has_task(target) is false at this point, as is
target__has_cpu(target)), because perf starts the workload later via
perf_evlist__start_workload.

So it looks like I shouldn't have any synthesized events. Have I missed
anything?

Thanks,
Mark.
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