Re: [PATCH 2/2] perf tools: Cope with sparsely-numbered CPUs
From: Ingo Molnar
Date: Wed Jan 20 2010 - 02:50:16 EST
* Anton Blanchard <anton@xxxxxxxxx> wrote:
> Hi Ingo,
> > > For system-wide monitoring, the perf tools currently ask how many CPUs are
> > > online, and then instantiate perf_events for CPUs 0 ... N-1. This doesn't
> > > work correctly when CPUs are numbered sparsely. For example, a four-core
> > > POWER6 in single-thread mode will have CPUs 0, 2, 4 and 6. The perf tools
> > > will try to open counters on CPUs 0, 1, 2 and 3, and either fail with an
> > > error message or silently ignore CPUs 4 and 6.
> > >
> > > This fixes the problem by making perf stat, perf record and perf top
> > > create counters for increasing CPU numbers until they have a counter
> > > for each online CPU. If the attempt to create a counter on a given
> > > CPU fails, we get an ENODEV error and we just move on to the next CPU.
> > > To avoid an infinite loop in case the number of online CPUs gets
> > > reduced while we are creating counters, we re-read the number of
> > > online CPUs when we fail to create a counter on some CPU.
> > >
> > > Reported-by: Michael Neuling <mikey@xxxxxxxxxxx>
> > > Tested-by: Michael Neuling <mikey@xxxxxxxxxxx>
> > > Cc: stable@xxxxxxxxxx
> > > Signed-off-by: Paul Mackerras <paulus@xxxxxxxxx>
> > > ---
> > > tools/perf/builtin-record.c | 36 ++++++++++++++++++++++++++++--------
> > > tools/perf/builtin-stat.c | 15 +++++++++++++--
> > > tools/perf/builtin-top.c | 27 +++++++++++++++++++--------
> > > 3 files changed, 60 insertions(+), 18 deletions(-)
> > nice fix!
> > The linecount bloat is a bit worrying though. I'm wondering, since we have 3
> > loops now (and possibly more upcoming), wouldnt it be a cleaner fix to have
> > some generic idiom of 'loop through all online cpus' somewhere in lib/*.c?
> > This would work better in the long run than spreading all the sysconf calls
> > and special cases across all those callsites. (new tools will inevitably get
> > it wrong as well)
> > As a practical matter we can commit your fix and do the cleanup/consolidation
> > as a separate patch, to not hold up your fix (and to help fix/bisect any
> > problems that would happen due to the consolidation) - as long as a
> > consolidation patch is forthcoming as well.
> It looks like this hasn't made it to mainline. Any chance we could get it in
> and look at a cleanup post 2.6.33?
Sure, if they come together we can apply one to an urgent branch and the other
to a .34 branch.
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