Re: perf script: rwtop: SIGALRM and pipe read race

[Andrew Jones]

On Mon, Sep 17, 2012 at 08:55:43AM -0600, David Ahern wrote:
> On 9/14/12 12:10 PM, Andrew Jones wrote:
> >On Fri, Sep 14, 2012 at 10:05:03AM -0600, David Ahern wrote:
> >>On 9/14/12 9:39 AM, Andrew Jones wrote:
> >>>
> >>>I recently tried 'perf script rwtop', and it immediately failed with
> >>>'failed to read event header'. Running it through strace I found that the
> >>>when rwtop.pl is reading from the pipe, and gets one of it's alarms, that
> >>>the ERESTARTSYS seems to confuse it - causing it to fail. It also appears
> >>>that the problem only happens early in execution, or not at all. If I get
> >>>lucky and don't hit the problem right away, then rwtop will run fine as
> >>>long as I want, without any ERESTARTSYS's in its trace. I also found that
> >>>I can avoid hitting the problem by throwing a 'pv -q' in front of the perf
> >>>command in tools/perf/scripts/perl/bin/rwtop-report. Which I guess slows
> >>>things down in the reader enough to always avoid the race.
> >>>
> >>>Sorry I don't have a solution (patch). I'll look at it more as time
> >>>permits, but I thought I'd get it reported for starters though.
> >>
> >>
> >>This fixes the run-time problem:
> >>
> >>diff --git a/tools/perf/util/util.c b/tools/perf/util/util.c
> >>index 1b8775c..a4371ae 100644
> >>--- a/tools/perf/util/util.c
> >>+++ b/tools/perf/util/util.c
> >>@@ -142,6 +142,9 @@ int readn(int fd, void *buf, size_t n)
> >>     while (n) {
> >>         int ret = read(fd, buf, n);
> >>
> >>+        if ((ret < 0) && (errno == EINTR))
> >>+            continue;
> >>+
> >>         if (ret <= 0)
> >>             return ret;
> >>
> >>
> >>
> >>The only problem you will find with rwtop is that bytes_read will be
> >>really whacky. I traced it to:
> >>
> >>     if ($ret > 0) {
> >>printf("comm %s bytes_read %d\n", $common_comm, $ret);
> >>         $reads{$common_pid}{bytes_read} += $ret;
> >>
> >>Somehow the $ret > 0 is passing when in fact it is negative. I do
> >>not know much about perl to fix it.
> >>
> >
> >This actually appears to be an issue with how perl sighandlers are
> >supposed to work.
> >
> >http://perldoc.perl.org/perlipc.html#Restartable-system-calls
> >
> >I tried the below patch though, and while it gets me past the read failure
> >it still doesn't solve the problem. With it the script stops processing
> >events after the first one.
> >
> >Drew
> >
> >diff --git a/tools/perf/scripts/perl/rwtop.pl
> >b/tools/perf/scripts/perl/rwtop.pl
> >index 4bb3ecd..8b20787 100644
> >--- a/tools/perf/scripts/perl/rwtop.pl
> >+++ b/tools/perf/scripts/perl/rwtop.pl
> >@@ -17,6 +17,7 @@ use lib
> >"$ENV{'PERF_EXEC_PATH'}/scripts/perl/Perf-Trace-Util/l
> >  use lib "./Perf-Trace-Util/lib";
> >  use Perf::Trace::Core;
> >  use Perf::Trace::Util;
> >+use POSIX qw/SIGALRM SA_RESTART/;
> >
> >  my $default_interval = 3;
> >  my $nlines = 20;
> >@@ -90,7 +91,10 @@ sub syscalls::sys_enter_write
> >
> >  sub trace_begin
> >  {
> >-    $SIG{ALRM} = \&set_print_pending;
> >+    my $sa = POSIX::SigAction->new(\&set_print_pending);
> >+    $sa->flags(SA_RESTART);
> >+    $sa->safe(1);
> >+    POSIX::sigaction(SIGALRM, $sa) or die "Can't set SIGALRM handler:
> >$!\n";
> >      alarm 1;
> >  }
> >
> >Drew
> >
> 
> Not sure why you want to change the signal handling. The display
> routine appears to be working.

Please read the link I posted to the Perl documentation. The standard
Perl signal handling doesn't support SA_RESTART. The Perl developers
found it could lead to data corruption. The patch above attempts to
replace the standard signal handler with a safe one that supports
SA_RESTART. It has nothing to do with the display routine, and
everything to do with avoiding the need for your EINTR patch.

> 
> Arnaldo had pinged me about rwtop a couple of weeks ago -- thinking
> one of my patches broke it. After looking into it I see 3 problems
> with the rwtop scripts:
> 
> 1. readn can fail with EINTR and that needs to be handled. The
> earlier patch fixes that.

This patch is a hack that shouldn't be necessary with proper
automatic ERESTARTSYS handling (SA_RESTART). Actually, I'd argue that
the patch is also wrong because it handles EINTR at all. The errors
should be propagated. What if some caller in builtin-* would prefer
to not use SA_RESTART, and then to make its own decisions on EINTR?

> 
> 2. the rwtop.pl script is not handling negative return values ($ret
> < 0) properly -- the '$ret > 0' check is succeeding even though $ret
> is negative (e.g., -EAGAIN) leading to astronomical read values

Yeah, I see this problem too, and don't know what's up with it. The
real magic happens in the macro craziness in /usr/lib/perl5/CORE/,
but as far a perf goes, it looks like it should be right
perl_process_tracepoint() has

                } else { /* FIELD_IS_NUMERIC */
                        val = read_size(pevent, data + field->offset,
                                        field->size);
                        if (field->flags & FIELD_IS_SIGNED) {
                                XPUSHs(sv_2mortal(newSViv(val)));
                        } else {
                                XPUSHs(sv_2mortal(newSVuv(val)));
                        }
                }

> 
> 3. record by default uses the watermark so it does not push records
> to the report script until the watermark is reached. This has 2 side
> effects:
> 
> a. for systems doing few reads and writes it can take a while for
> enough records to be generated causing an apparent hang when the
> command is launched. disabling delay (-D argument to perf-record in
> rwtop-record) handles that, but that's not the right answer for busy
> systems.

Adding -D fixed my problem with rwtop only processing the first event.

> 
> b. the display will not be accurate (up to date) given that there
> are buffered records that have not been processed.
> 
> Really we need a periodic mode for record that would push all
> records every N timer interval. This way everything is pushed to the
> processing script in a timely manner.
> 
> Arnaldo: I believe 3 explains the hang you saw.
> 
> David
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